NM_000180.4(GUCY2D):c.1996C>T (p.Arg666Trp) is a missense variant that replaces arginine with tryptophan at amino acid p.666. This variant is present in gnomAD v.4.1.1 at a total allele frequency of 0.00001673, with 27 alleles / 1613832 total alleles, which is lower than the ClinGen LCA/eoRD VCEP PM2_Supporting threshold of <0.0004 (PM2_Supporting). This variant has been reported in at least 1 proband with early-onset severe retinal dystrophy who was homozygous for the variant (0.5 points, PMID: 40478561). This variant has also been reported in at least 1 proband with early-onset severe retinal dystrophy who was compound heterozygous with the NM_000180.4 (GUCY2D):c.2945del (p.Gly982ValfsTer39) variant suspected in trans (0.5 points, PMID: 29559409), which was previously classified pathogenic by the ClinGen LCA/eoRD VCEP (1.0 total points, PM3). At least one proband harboring this variant exhibits a phenotype including diagnosis of congenital stationary night blindness (0.5 pts), vision loss in dim light since early infancy (1 pt), normal retinal structure by optical coherence tomography (1 pt), a non-detectable rod isolating electroretinogram (0.5 pts), evidence of cone involvement on ERG (1 pt) and genotyping by exome sequencing that did not provide an alternative explanation for visual impairment (4 pts), which together are highly specific for GUCY2D-related recessive retinopathy (8 total points, PMID: 29559409, PP4_Moderate). The variant exhibited 0% enzymatic activity in a guanylate cyclase assay relative to the wild-type control, which is lower than the ClinGen LCA/eoRD VCEP PS3_Supporting threshold of ≤10% activity, indicating that it triggers a severe defect in protein function (PS3_Supporting, PMID: 33109612). The computational predictor REVEL gives a score of 0.479, which is below the ClinGen LCA/eoRD VCEP threshold of ≥0.644 and does not predict a damaging effect on RetGC-1 protein function. Additionally, the splicing impact predictor SpliceAI gives a score of 0.04, which is below the ClinGen LCA/eoRD VCEP recommended threshold of ≥0.2 and does not strongly predict an impact on splicing. In summary, this variant meets the criteria to be classified as Likely Pathogenic for GUCY2D-related recessive retinopathy based on the ACMG/AMP criteria applied, as specified by the ClinGen LCA/eoRD VCEP: PM2_Supporting, PM3, PP4_Moderate, and PS3_Supporting. (VCEP specifications version 1.0.0).