The c.775G>A variant in the glucokinase gene, GCK, causes an amino acid change of alanine to threonine at codon 259 (p.(Ala259Thr)) of NM_000162.5. This variant is predicted to be deleterious by computational evidence, with a REVEL score of 0.948, which is greater than the MDEP VCEP threshold of 0.70 (PP3) and is absent from gnomAD v2.1.1 (PM2_Supporting). 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). Another missense variant, c.776C>T, p.Ala259Val, has been classified as pathogenic by the ClinGen MDEP but has a greater Grantham distance than p.Ala259Thr (PM5_Supporting). This variant was identified in 12 unrelated individuals with hyperglycemia (PS4; internal lab contributors). Additionally, this variant segregated with diabetes/hyperglycemia, with 7 informative meioses in 6 families (PP1_Strong; internal lab contributors). 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% and a 3-generation family history of diabetes) (PP4_Moderate; internal lab contributors). In summary, c.775G>A meets the criteria to be classified as 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_Supporting, PS4, PP1_Strong, PP4_Moderate, PM2_Supporting.