The NM_000330.4(RS1):c.668G>A variant is a missense variant encoding the substitution of Cysteine with Tyrosine at position 223. This variant is absent from hemizygous individuals in gnomAD v4.1.0 (PM2_Supporting). The computational predictor REVEL gives a score of 0.966, which is above the ClinGen X-linked IRD VCEP threshold of >0.932 and predicts a damaging effect on RS1 function (PP3_strong). The computational splicing predictor SpliceAI gives a delta score of 0.01 for 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. The variant has been reported to segregate with retinal dystrophy through 1 meiosis in a family consisting of a mother and an affected son, however, the mother was apparently unaffected so PP1 was not met (PMID: 38317323). At least one proband harboring this variant exhibits a phenotype including the appearance of schisis and reduced visual acuity before age 13 years, which together are specific for X-linked retinoschisis (PMID: 22245991, PP4). This variant has been reported in at least 3 apparently unrelated probands meeting the PS4 requirement of a male diagnosed with X-linked retinoschisis (PMIDs: 20061330, 38317323, 39293640, PS4_moderate). This variant p.Cys223Tyr disrupts a site defined as a critical amino acid residue involved in disulfide bridge formation by the ClinGen X-linked IRD VCEP (PMIDs: 26812435). The PM1_strong code is ineligible to be used in combination with the PP3 code, and therefore, this code was not met. Another missense variant in the same codon, NM_000330.4(RS1):c.667T>C (p.Cys223Arg), (PMIDs: 34645606, 10533068, 30652005, 33460243, 16361673) has been classified as pathogenic for X-linked retinoschisis by the ClinGen X-linked IRD VCEP, while no benign missense variants have been identified in this codon. The present variant has a higher Grantham’s distance (194) than the comparison variant (180), and SpliceAI has been used to confirm that neither variant has a predicted impact on RS1 splicing (PM5). 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_strong, PP4, PM5, and PS4_moderate (date of approval 01/24/2025).