The c.775G>T variant in the glucokinase gene, GCK, causes an amino acid change of alanine to serine at codon 259 (p.(Ala259Ser)) of NM_000162.5. This variant is predicted to be deleterious by computational evidence, with a REVEL score of 0.744, which is greater than the MDEP VCEP threshold of 0.70 (PP3). GCK is defined by the ClinGen MDEP as a gene that has a low rate of benign missense variation and has pathogenic missense variants as a common mechanism of disease (PP2). Two other missense variants, c.776C>T p.Ala259Val and c.775G>A p.Ala259Thr, have been interpreted as pathogenic by the ClinGen MDEP, and p.Ala259Ser has a greater Grantham distance than p.Ala259Val and p.Ala259Thr (PM5_Strong). This variant was identified in an individual with a clinical history highly specific for GCK-hyperglycemia (FBG 5.5-8 mmol/L and HbA1c 5.6 - 7.6%) (PP4; PMID: 30245511). This variant segregated with hyperglycemia/diabetes with one informative meiosis in a single family; however, this does not meet the thresholds for PP1 set by the ClinGen MDEP (PMIDs: 27236918, 30245511). In summary, c.775G>T meets the criteria to be classified as likely pathogenic for monogenic diabetes. ACMG/AMP criteria applied, as specified by the ClinGen MDEP (specification version 1.3.0, approved 8/11/2023): PP3, PP2, PM5_Strong, PP4.