Individuals diagnosed with primary brain tumors (PBTs) experience major morbidity and mortality, compounded by the risks associated with standard-of-care brain-directed therapeutic approaches. Recurrence remains inevitable in patients with high-grade gliomas (HGG), due to their brain-infiltrative growth hindering surgical removal, immunosuppressive tumor-microenvironment, dynamic disease evolution from their high cellular and molecular heterogeneity, and multi-faceted challenges to brain drug delivery, primarily stemming from the blood-brain barrier (BBB). This expert narrative review describes the current state of transcranial focused ultrasound (FUS), and its multi-modal applications for PBTs, including delivery of therapeutic agents and sono-liquid biopsy (via BBB opening), immunomodulation, radio-sensitization, and direct destruction of tumor cells/tissue via thermoablation, histotripsy, and sonodynamic therapy. BBB opening-based approaches have the most promising clinical evidence so far, warranting randomized comparative evalutions. Further translation will require standardized FUS treatment protocols, translational investigations nested into trials, coordinated global efforts, strategic trial design incorporating methodological advances, and implementation approaches enabling broader participation. Worldwide efforts to advance FUS will be aided by ongoing device evolution, support from professional societies, and the development of FUS research consortia (like ReFOCUSED). FUS applications open a potential combinatorial path forward with systemic therapies for "adaptive theragnostic" tumor management, thus targeting the root causes of therapeutic failure for HGG patients.
Reirradiation has re-emerged as a viable salvage modality for recurrent central nervous system (CNS) tumors in the modern era of precision radiotherapy. Advances in image guidance, biological understanding of normal-tissue repair, and integration of targeted systemic agents have expanded its therapeutic window. The 2025 ISRS meta-analysis (62 studies; n = 2640) established a median overall survival (OS) of ~ 10.2 months after re-irradiation (re-RT) of recurrent HGG, confirming that focal single-fraction SRS (16 Gy × 1) or hypofractionated SRS (25 Gy/5 fx) achieves optimal tumor control with acceptable neurologic toxicity (7% vs 4%). EQD₂ > 120-130 Gy markedly increases radionecrosis risk (p = 0.003). Integration of DNA-damage-repair (DDR) inhibitors, VEGF blockade, and tumor-treating fields (TTFields) is under active investigation. Re-RT in ependymoma, meningioma, and spinal tumors is also gaining renewed interest with proton and MR-guided adaptive techniques.
The inevitable progression of high-grade gliomas has prompted a need for data-backed identification of compromised tissue prior to detection on traditional serial imaging. Whole-brain magnetic resonance spectroscopy (WB-MRS) can fill this role to classify glioma progression prior to contrast enhancement. Voxel-level data can differentiate areas of perilesional tissue under supervised machine learning (ML) and has been shown to predict the likelihood of tumor progression within six months. In this study, we aim to improve the spatial utility of WB-MRS ML through unsupervised ML clustering for utility in intraoperative integration with existing neuronavigation platforms. This analysis involved 16 adult patients that developed recurrence of high-grade glioma (HGG) on serial imaging, including 13 with glioblastoma (GBM) and three with anaplastic astrocytomas. Postoperative WB-MRS images were used as data inputs. We investigated two unsupervised clustering methods to optimize the identification of compromised perilesional tissue from a previously published supervised model. All voxels within the new region of interest (ROI) are reclassified as future progression regardless of their classification from the supervised model. After hyperparameter tuning, Density-Based Spatial Clustering of Applications with Noise (DBSCAN) shows an area under the curve (AUC) of 0.942, while K-Means Clustering shows an AUC of 0.874. DBSCAN also shows a superior accuracy, specificity, sensitivity, and F1-score to the original supervised model alone. The incorporation of unsupervised clustering enhances the utility of WB-MRS for identifying compromised perilesional tissue and predicting high-grade glioma progression. Unsupervised clustering incorporates critical geospatial data and offers a more surgically relevant approach to visualizing tumor progression. Early detection of high-grade glioma recurrence remains a major limitation of current imaging modalities, restricting the ability to guide timely and precise interventions. While prior work has demonstrated that WB-MRS combined with supervised ML can identify metabolically abnormal tissue at risk for progression, these approaches lack spatial coherence and clinical usability. In this study, we introduce a hybrid pipeline integrating supervised voxel-wise predictions with unsupervised spatial clustering (DBSCAN and K-means) to refine recurrence mapping. This approach significantly improves classification performance and generates spatially contiguous, clinically interpretable regions of interest. Importantly, these outputs can be exported as DICOM overlays, enabling direct integration into neuronavigation systems. While still in pilot phase, this work advances WB-MRS from a predictive tool toward a clinically integrable platform, with potential applications in surgical planning, biopsy targeting, and longitudinal disease monitoring in patients with high-grade glioma.
Prosody perception is an often overlooked aspect of human language despite its importance in facilitating spoken language comprehension. Sensitivity to prosodic cues varies between individuals, and prosody perception skills are shown to be associated with various language- and reading-related outcomes. Despite the importance of prosody perception in human communication, its underlying biology is poorly understood. This study investigates the genetic architecture of prosody (speech rhythm) perception and explores its evolutionary roots. We conducted a GWAS of prosody (n = 1,501) as measured by scores on the Test of Prosody via Syllable Emphasis ("TOPsy"). GWAS results yielded 14 suggestive significant signals (p < 5.00 × 10-6). Gene set enrichment analysis identified shared genetic architecture between human prosody perception and key vocal learning brain regions in songbirds, suggesting that human prosody perception may have evolutionary convergence in communication mechanisms in animal vocal learning. Additionally, cross-trait polygenic score analyses suggest shared genetic influences between prosody perception and both word reading and musical beat synchronization, emphasizing how genetics influence prosody perception and its associations with communication-, education-, and music-related traits. These initial efforts could inform advances in communication sciences and disorders as well as educational contexts.
Inherited retinal diseases (IRDs) comprise a diverse group of disorders that frequently lead to progressive vision impairment and blindness. Despite advances in genetic testing, a significant number of IRD cases remain genetically unsolved, often due to unidentified disease-associated genes or variants. This study reports additional cases for the newly discovered IRD genes of the AP-5 complex. A comprehensive ophthalmological evaluation was performed for all patients, including retinal imaging (multimodal imaging), visual field testing, and electroretinogram (ERG) testing. Whole-genome and -exome sequencing (WGS and WES) were performed for clinically unsolved IRD patients, and data were analyzed to identify underlying causal variants. The identified variants were subsequently validated using Sanger sequencing. Five unrelated patients from Europe and Iran were identified with a distinctive macular degeneration associated with bi-allelic variants in AP5Z1 (HGNC: 22197) and AP5B1 (HGNC: 25104), subunits of the vesicular fifth adaptor protein (AP-5) complex. The AP-5 complex is the part of the intracellular trafficking machinery thought to be involved in cellular homeostasis and lysosomal functioning in the retinal pigment epithelium (RPE). The identification of bi-allelic variants in two proteins of the AP-5 complex expand the characterization of AP-5 genes in sustaining and preserving normal macular function.
Bicuspid aortic valve (BAV) is the most common congenital heart lesion in adults and is often associated with thoracic aortic aneurysms and aortic stenosis. The genetic causes of most non-syndromic cases of BAV remain unknown. Pathogenic variants in COL1A1 or COL1A2, which encode type I collagen (COL1), cause osteogenesis imperfecta (OI), a rare disorder marked by bone fragility. Although aortic valve phenotypes, including BAV, have been described in OI, COL1 genes have not been implicated in sporadic BAV. We report rare COL1 variants in individuals with early-onset BAV (EBAV) complications. Whole-exome sequencing was performed in 272 non-syndromic BAV probands who developed valve or aortic complications before age 30 (EBAV) and 272 biological relatives. Variants were filtered by allele frequency, inheritance pattern, ClinVar classification, and in silico predictions. Participants with rare predicted damaging COL1 variants were recontacted to confirm phenotypes. Rare coding variants in COL1A1 (n = 5) and COL1A2 (n = 5) were identified in 10 (4%) EBAV probands, representing a 30-fold enrichment compared with European ancestry populations. Only two variants involved glycine substitutions in triple-helical domains. Affected probands presented with aortic regurgitation and/or thoracic aneurysms requiring repair and exhibited subtle connective tissue features such as joint hypermobility, recurrent fractures, or dental abnormalities. Predicted damaging COL1 variants are enriched in EBAV probands with features overlapping OI and Ehlers-Danlos syndrome, though involving different residues than those in OI. Genetic testing for these variants may help identify individuals who could benefit from personalized surveillance or targeted preventive strategies.
The impact of clinical exome and genome sequencing (ES/GS) depends on the clinical setting. In the sequencing arm of a multifactor randomized clinical trial to evaluate broadening access, we assessed the diagnostic and inconclusive findings of ES as a "first-tier" test in 101 pediatric participants in whom suspicion of a genetic condition by primary care providers had prompted initial outpatient consultation in a pediatric genetics or neurology clinic. This implementation focused on the early stages of the diagnostic odyssey, capturing a clinically less-selected population with lower pre-test probability than traditional specialist cohorts. Variants were prioritized using phenotype-driven gene lists. After returning the results to participants, the clinical teams performed familial variant testing or additional phenotyping at their discretion, based on potential diagnostic benefit. We then implemented a multidisciplinary case conference in which the laboratory and clinical teams assessed additional clinical information. Initially, 57% of participants had non-negative reports: 5% had one or more variant findings considered explanatory for the presenting phenotype, with 52% having results initially classified as inconclusive. After family testing and/or phenotypic characterization, a total of 9% were considered positive/diagnostic, 10% were reclassified from inconclusive to negative, and 38% remained inconclusive. While ES, as a first-tier genetic test, can expedite some diagnoses, these results demonstrate challenges in early implementation. In this population, initial testing can leave substantial residual uncertainty, shifting the diagnostic odyssey rather than concluding it and necessitating factors such as parental testing, ongoing phenotyping, and reassessment of variants over time to resolve inconclusive results.
Germline pathogenic variants that activate the Ras/mitogen-activated protein kinase (MAPK) pathway cause neurodevelopmental disorders called Rasopathies. Because many affected proteins directly regulate Ras, causative mutations may alter other Ras-dependent pathways in addition to MAPK signaling. To better understand which Rasopathy sequelae result from hyperactivation of downstream MAPKs, we engineered mice with a gain-of-function mutation in the terminal MAPK gene Mapk1, which encodes ERK2 and is associated with the recently described genetic syndrome MAPK1-related Rasopathy (MRR). Mapk1 mutant mice successfully modeled key aspects of the human MRR phenotype, including small stature, facial dysmorphism, and impaired cognitive function. Importantly, they recapitulated phenotypes identified in Rasopathy models with upstream Ras activation, such as neurofibromatosis type 1 (NF1): oligodendrocyte lineage defects, reactive astrogliosis, memory deficits, and hypersensitivity to sensory stimuli. These findings emphasize the importance of downstream MAPK signaling in the pathophysiology of neurocognitive symptoms observed in Rasopathy syndromes.
We report three individuals with bi-allelic variants in RNU6ATAC, which encodes the U6atac minor spliceosomal small nuclear RNA (snRNA), causing a multisystem minor spliceopathy. Through RNA sequencing analysis, we identified a distinctive excess of minor intron retention (MIR) in two unrelated individuals, which guided the identification of bi-allelic RNU6ATAC variants. The discovery cohort presented with variable multisystem manifestations. One individual presented with refractory epilepsy, microcephaly, developmental delay, ataxia, bilateral toe syndactyly, hypereosinophilia, and short stature, whereas the other exhibited failure to thrive, short stature, primary hypothyroidism, combined variable immunodeficiency, eosinophilic colitis, ichthyosis vulgaris, scoliosis, and chronic inflammatory demyelinating polyneuropathy without neurodevelopmental involvement. Despite organ-specific variation, both individuals displayed impaired growth and eosinophil-driven inflammation. Recently, we identified a third affected individual from an independent cohort whose phenotype bridges these features, combining microcephaly, growth failure with severe immunodeficiency, and skeletal abnormalities. The distinctive excess of MIR outliers in the discovery cohort supports minor spliceosome dysfunction, mirroring the molecular signature of RNU4ATAC-opathy. These findings nominate RNU6ATAC as a disease-associated gene, defining an expanded clinical spectrum of minor spliceopathies. Our study supports the power of integrating genomic and transcriptomic approaches for diagnosing splicing disorders and highlights the critical role of spliceosomal snRNAs in human disease.
Autosomal-dominant (AD) pathogenic/likely pathogenic (P/LP) variants in von Hippel-Lindau (VHL) gene cause VHL disease. We characterize VHL variants and disease phenotypes in Black/African American (AA) patients, a demographic that has not been thoroughly studied. Black/AA patients undergoing germline genetic testing at a CLIA-certified commercial laboratory from November 2014 to March 2022 and carrying a P/LP AD VHL gene variant were identified. Personal and family histories were obtained from test requisition forms, and patients were categorized into VHL disease subtypes: type 1, type 2A, type 2B, and type 2C. Patients with a personal or family history of cancer but no lesions typical of VHL disease were categorized as "unclassified." Patients with an incomplete personal or family history available were noted as "unknown." Final analysis included 38 patients. Among the cohort's personal cancer history, hemangioblastoma and pheochromocytoma were most prevalent (16%). Among the cohort's family cancer history, renal cell carcinoma was most prevalent (8%). Type 1 was the most common VHL disease class recorded (34%). Substitution variants were most common (76%); p.Arg167Trp (c.499C>T) was the most common substitution (8%). Unique variants in Black/AA patients include p.Pro81Leu (c.242C>T), p.Leu129Pro (c.386T>C), p.Asp121Val (c.362A>T), p.His110Profs∗49 (c.329del), and p.Arg82His (c.245G>A). This dataset informs future research on VHL disease and treatment in Black/AA populations.
Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease with multiple genetic causes. Given the strong evidence of mitochondrial dysfunction in ALS, this study aimed to identify genetic contributors to ALS by focusing on genes involved in mitochondrial function. Whole-genome and whole-exome sequencing data from 1,034 individuals with ALS were analyzed using two distinct computational tools, which ranked candidate genes based on functional relevance to ALS. POLG, the sole mitochondrial DNA (mtDNA) polymerase, emerged as a top candidate gene. RNA sequencing (RNA-seq) analysis revealed that among genes upregulated in samples with a POLG variant, there was an enrichment for mitochondrial pathways, including translation, localization, and mitophagy. It also revealed variants in POLG and SOD1, a well-known ALS gene, to be the most enriched in samples with expression profiles of mitochondrial-related genes that differed most from those of unaffected control subjects. POLG variant carriers also exhibited an increased burden of mitochondrial genome variants, a pattern shared by carriers of variants in other genes involved in mtDNA maintenance. Additionally, POLG variant carriers had elevated mtDNA copy number (mtDNA-CN), similar to carriers of variants in mitophagy-related genes, suggesting impaired mitophagy. Together, these findings implicate POLG as an ALS-associated gene and link mtDNA maintenance defects, altered expression of mitochondrial-related pathways, and impaired mitophagy to the ALS etiology.
DSCAM occupies a 1-Mb locus in the original Down syndrome critical region on chromosome 21q22 and encodes a neuronal cell adhesion molecule of importance for brain and eye development. Singleton individuals, both born to first-cousin parents, with intellectual disability and homozygous DSCAM loss-of-function variants were reported in 2017 and in 2021, the latter also presenting with nystagmus and visual impairment. We present a cohort of five individuals, four new, including two sibling pairs with homozygosity or compound heterozygosity for predicted loss-of-function DSCAM variants. We identify a common clinical pattern of moderate to severe neurodevelopmental delay with poor language development, risk of focal seizures with onset in infancy, and nystagmus with poor vision. Electroretinography in two of the affected revealed cone-pathway dysfunction with a b-wave pattern indicating main dysfunction at the level of the cone-associated bipolar cells of the central retina. Our electroclinical findings are in line with previous DSCAM knockout chicken and mice studies that evidenced disturbed horizontal and vertical patterning of the retina. Taken together, we delineate a rare syndromic form of recessive intellectual disability with a distinctive type of visual impairment.
Mitochondrial disorders show remarkable clinical and genetic heterogeneity and result from variants in either mitochondrion- or nucleus-encoded genes. CHCHD4 is a component of the mitochondrial import and assembly pathway that imports small cysteine-containing substrates. We report a pediatric patient with biallelic CHCHD4 variants who presented with severe neurological regression and early death. Western blot analysis showed decreased levels of CHCHD4 and diminished assembly of complexes I and IV in his fibroblasts. To demonstrate that CHCHD4 variants were responsible for the observed biochemical phenotype, we overexpressed wild-type CHCHD4 in control and subject fibroblasts, restoring levels of complex I and IV proteins and the associated assembly defects. Proteomic studies pointed to electron transport and complex I biogenesis as the main dysregulated pathways and showed a severe loss of several complex I and IV proteins and/or assembly factors rescued by overexpression of wild-type CHCHD4. CHCHD4 has numerous targets and interacting factors and is involved in the export of iron-sulfur clusters synthesized inside mitochondria. Surprisingly, few of these interacting factors or non-mitochondrial functions were impacted by the observed CHCHD4 defect. In conclusion, our work establishes CHCHD4 deficiency as a cause of dysregulated mitochondrial protein import resulting in a severe neurological condition.
Pathogenic missense variants in PRKACA cause craniofacial, skeletal, and cardiac defects similar to Ellis-van Creveld syndrome. We report an individual with a previously unreported, de novo 3-amino-acid deletion in PRKACA, identified on trio genome sequencing, and phenotypic features including severe neonatal hypotonia, appendicular skeletal abnormalities, osteopenia, aortic dilation, coronary dilation, and vascular tortuosity. To assess this variant's effects, we performed in vitro and in vivo studies, generated an in silico model, and assessed cell ciliation in induced pluripotent stem cells from the patient. Although the protein product of the PRKACA (Protein kinase A [PKA]-Cα) 3-amino-acid-deletion variant is catalytically active, it shows reduced interaction with the regulatory subunits of PKA (particularly type II), resulting in overactivation of the PKA pathway and/or an inability to initiate Hedgehog signaling. The deletion affects a key portion of PKA-C important for substrate tethering. Patient-derived induced pluripotent stem cells (iPSCs) have reduced ciliation compared to controls. Collectively, this supports that the PRKACA variant is pathogenic, and we propose that it is causal for our patient's unique skeletal dysplasia and vasculopathy phenotypes. This expands the phenotypic spectrum of pathogenic variants in PRKACA and suggests that affected individuals may require periodic screening for aortic and coronary dilation as well as osteopenia.
Index trait bias (also called index event bias) can occur in genetic studies due to conditioning on incident trait, which can bias genetic associations with subsequent traits. We propose the use of two Bayesian Mendelian randomization (MR) methods (Bayesian weighted MR [BWMR] and MR-HORSE) to correct index trait bias in genome-wide association studies (GWASs) of subsequent traits. We compare these Bayesian MR methods to previously proposed methods for index trait bias through a simulation study. We observe that BWMR has similar type I error compared to using an inverse variance weighted MR, weighted median MR, and Dudbridge but has an inflated type I error compared to SlopeHunter. MR-HORSE and SlopeHunter have similar type I errors for smaller correlations between incident and subsequent traits; however, MR-HORSE and SlopeHunter have better controlled type I error for a large negative correlation and large positive correlation, respectively. All methods have comparable power across correlations between incident and subsequent traits. We applied the methods to a GWAS of subsequent acute ischemic stroke (AIS) or 3-point major adverse cardiovascular event after an incident AIS event in the Million Veteran Program and for fasting insulin adjusted for body mass index, and we observed slight differences in the results between the correction methods. We observed that a single index trait bias correction method is not optimal across all scenarios; therefore, applying multiple methods and checking for consistency between the estimates could provide an approach to determine the presence of and correction for index trait bias.
Autonomy is a leading indicator of well-being and a cornerstone principal of research ethics. Yet adults with intellectual disability are routinely denied autonomy in research settings, affecting participation and generalizability. To inform health research policy and practice, we conducted a national survey of 150 adults with intellectual disability to explore their views on autonomy and decision-making in precision medicine research (PMR), a leading area of health research. Participants expressed strong interest in participating in PMR and controlling their participation decisions, sometimes with support. Findings highlight the need for policies and practices that honor these interests and facilitate autonomy. Over time, such efforts can improve health outcomes for this underserved population.
The melanocortin 1 receptor (MC1R) is one of the fundamental proteins regulating skin and hair color in mammals. In India, the red hair color (RHC) phenotype is extremely rare. We analyzed MC1R and identified an ultra-rare pathogenic variant, c.872C>A (p.Ala291Asp), that is responsible for the RHC phenotype in an Indian child. Further, we screened the complete coding region of MC1R in a total of 11,021 individuals, representing 91 distinct Indian populations across India, and found a total of 21 novel or ultra-rare variants. In silico, in vitro, and zebrafish-based functional analysis showed that c.742G>T (p.Gly248Cys) and p.Ala291Asp variants lead to loss-of-function (LoF) effects. The distribution of some of these variants differed significantly among Indian populations. One of them is MC1R c.-226A>T (rs3212363), which is significantly associated with lighter skin pigmentation in the Bodh population inhabited in Ladakh. On average, TT homozygotes were 8.46 melanin units lighter compared with AA homozygotes (95% confidence interval [CI]: 3.211 to 13.72; adjusted p = 0.0005). Our study identifies and functionally validates ultra-rare MC1R variants as potential causes of RHC phenotype in Indians.
Rare genetic variation is considered a potential source of heritability in individuals with sporadic Alzheimer disease and related dementias (ADRD). The Variant-set test for association using annotation information (STAAR) framework leverages multiple functional annotations of genetic variants and combines association statistics from multiple variant aggregation-based methods, including burden, sequence kernel association test (SKAT), and aggregated Cauchy association test (ACAT-V), into a single measure of significance. Using whole-genome sequencing data from the Alzheimer's Disease Sequencing Project (ADSP), we comprehensively examined the association of rare genetic variation with ADRD in 23,454 individuals (37% individuals affected by ADRD) and with cognitively healthy elder status in 13,292 individuals (13% cognitively healthy elders) from diverse populations via the STAAR framework. We identified several genes significantly associated with ADRD or cognitively healthy status. However, our analysis revealed several limitations within the STAAR framework incorporating ultra-rare variants with dichotomous outcomes. To enhance the robustness of the framework, we proposed several computational refinements, including creating a burden of ultra-rare variants and employing more precise annotations to match the expected mechanism. After implementing the proposed modifications, the association with ADRD for ZNF200 was no longer statistically significant (α = 1 × 10-7), while TBX19, PLXNB2, CARD11, and LINC01880 remained significantly associated with cognitively healthy status. We identified and addressed the computational limitations in the STAAR framework that could lead to potential spurious results for ultra-rare variant aggregates with an extremely low cumulative minor-allele count. Our proposed refinements produced more robust results for associations with rare variants in the context of dichotomous outcomes.
Heterotaxy (HTX) is a congenital disorder characterized by abnormal left-right organ placement, often leading to severe congenital heart disease (CHD). Despite advances in sequencing, many CHD and HTX-associated genes remain functionally unvalidated, hindering effective clinical diagnosis and management. Here, we leveraged a high-throughput CRISPR-Cas9 screening approach in the Xenopus model to rapidly evaluate candidate genes identified from whole-exome sequencing of human CHD patients. Our screen identified Filamin B (FLNB), an actin-binding protein previously linked to skeletal disorders but not to ciliopathies or CHD. We identified 5 probands with CHD and HTX, 3 with recessive and 2 with damaging heterozygous variants in FLNB. Disrupting flnb in Xenopus reproduced key features of the human HTX phenotype, including defects in cardiac development and impaired motile cilia function. Rescue experiments confirmed the functional conservation of human FLNB, directly implicating actin cytoskeletal disruption in ciliogenesis and left-right patterning defects. Our results provide crucial evidence linking human FLNB dysfunction to ciliopathies and CHD and HTX.
X-chromosome inactivation (XCI) modifies disease severity in females with X-linked variants, but clinically applicable high-resolution assessment remains limited. We report a family with an AIFM1 variant showing marked intrafamilial phenotypic variability and evaluated whether haplotype-resolved nanopore sequencing can inform clinical interpretation. Targeted long-read sequencing was performed in a severely affected hemizygous male, his asymptomatic heterozygous mother, and a severely affected heterozygous sibling. In the hemizygous male, the sample served as a technical control, with all reads mapping to a single haplotype, consistent with a hemizygous X chromosome. Among heterozygous carriers with the identical variant (c.506C>T; p.Pro169Leu), XCI correlated with severity: the affected sibling showed 84% skew favoring activation of the pathogenic allele, whereas the mother showed preferential inactivation (20%). This family-based study shows that using nanopore sequencing for haplotype-resolved X-inactivation (XCI) analysis may provide a practical framework for selected X-linked disorders with variable expressivity.