The NM_000330.4(RS1):c.266A>G (p.Tyr89Cys) variant is a missense variant encoding the substitution of Tyrosine and Cystine at amino acid 89. This variant is absent from hemizygous individuals in gnomAD v4.1.0 (PM2_Supporting). The computational predictor REVEL gives a score of 0.912, which is within the ClinGen X-linked IRD VCEP range between 0.931 and 0.773 and predicts a damaging effect on RS1 function (PP3_moderate). The computational splicing predictor SpliceAI gives a delta score of 0.01 for donor loss and acceptor gain, which is below the ClinGen X-linked IRD VCEP threshold of >0.2 and does not predict that the variant disrupts RS1 splicing. COS-7 cells exogenously expressing the variant exhibit loss of RS1 secretion into the medium relative to the wild-type control (PMID: 16361673, PS3_Supporting). At least one proband harboring this variant exhibits the following phenotypes, including the appearance of schisis and reduced visual acuity before age 13 years and showing retinal detachment, which together are specific for X-linked retinoschisis (PMID: 17987333, PP4_Moderate). This variant has been reported in at least 3 apparently unrelated probands meeting one of the PS4 requirements of a male diagnosed with XLRS (PMIDs: 10220153, 11035549, 30652005, 19324861, 30551202, PS4_Moderate). The variant has been reported to segregate with retinal dystrophy through >3 meioses in two families (PP1_Strong; PMID: 17987333, 19324861). In summary, this variant is classified as pathogenic for X-linked retinoschisis based on the ClinGen X-linked Inherited Retinal Disease Expert Panel Specifications to the ACMG/AMP Variant Interpretation Guidelines for RS1 Version 1.0.0: PM2_Supporting, PP3_moderate, PS3_Supporting, PP4_Moderate, PS4_Moderate, and PP1_Strong.