NM_000330.4(RS1):c.655T>C (p.Cys219Arg) is a missense variant encoding the substitution of cysteine with arginine at amino acid 219. Another missense variant in the same codon, NM_000330.4(RS1):c.655T>G (p.Cys219Gly) has been classified as likely 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 (180) than the comparison variant (159), and SpliceAI has been used to confirm that neither variant has a predicted impact on RS1 splicing (PM5_Supporting). This variant is absent from hemizygous individuals in gnomAD v4.1.0 (PM2_Supporting). This variant has been reported in at least 2 unrelated probands meeting the PS4 requirement of a male diagnosed with X-linked retinoschisis (PMID: 39462066, PMID: 35982512, PMID: 9618178, PS4_Supporting). The variant has been reported to segregate with X-linked retinoschisis through 2 affected family members (PP1; PMID: 39462066). This variant has been identified as a de novo occurrence with unconfirmed parental relationships in 1 individual meeting the PS2 phenotype requirement of a male diagnosed with retinoschisis (PMID: 9618178). No description was provided of parental genotyping, so PS2 was not met. COS-1 cells exogenously expressing the variant protein exhibit loss of RS1 secretion into the medium relative to the wild-type control, caused by misfolding, aggregation, and ER retention of the variant protein (PMID: 12746437, PS3_Supporting). The computational predictor REVEL gives a score of 0.978, 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, which is below the ClinGen X-linked IRD VCEP threshold of ≥0.2 and does not predict that the variant disrupts RS1 splicing. 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, PM5_Supporting, PS3_Supporting, PS4_Supporting, and PP1.