NM_000330.4(RS1):c.655T>G (p.Cys219Gly) is a missense variant encoding the substitution of cysteine with glycine at amino acid 219. This variant is absent from hemizygous individuals in gnomAD v4.1.0 (PM2_Supporting). This variant has been reported in at least 1 proband meeting the PS4 requirement of a male diagnosed with X-linked retinoschisis (PMID: 9618178, PMID: 15937075). However, PS4_Supporting requires at least 2 unrelated probands, so this criterion was not met. The computational predictor REVEL gives a score of 0.984, 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 0.00 for all predictive splice changes, which is below the ClinGen X-linked IRD VCEP threshold of ≥0.2 and does not predict that the variant disrupts RS1 splicing. Exogenously expressed RS1 harboring the variant was subjected to gel filtration-based separation and PAGE analysis on a non‐reducing gel and exhibits defective oligomerization indicating no formation of octamers (PMID: 16361673, PS3_Supporting). This variant is a missense substitution affecting a critical amino acid residue involved in disulfide bridge formation as defined by the ClinGen X-linked IRD VCEP (PMID: 26812435). PM1_Strong is not met as this code is ineligible to be used in combination with PP3_Strong. In summary, this variant is classified as likely 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, and PS3_Supporting.