The c.782G>A variant in the glucokinase gene, GCK, causes an amino acid change of glycine to glutamic acid at codon 261 (p.(Gly261Glu)) of NM_000162.5. 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). This variant is predicted to be deleterious by computational evidence, with a REVEL score of 0.989, which is greater than the MDEP threshold of 0.70 (PP3). This variant is absent in gnomAD v2.1.1 (PM2_Supporting). This variant was identified in more than 11 unrelated individuals with diabetes/hyperglycemia (PS4; PMID 19358091, 8168652 , internal lab contributors). This variant segregated with diabetes/hyperglycemia with 11 informative meioses in 4 families (PP1_Strong; internal lab contributors). A kinetic analysis of recombinant wild-type (WT) and mutant glucokinase demonstrated that the wild-type kinetic parameters pass the quality control, the wild-type ATP Km is between 0.4-0.65, and the p.Gly261Glu variant has a relative activity index (RAI) of 0, which is less than the MDEP VCEP threshold of 0.50 (PMID: 19903754). Another missense variant, c.781G>A, (p.Gly261Arg), has been classified as pathogenic by the ClinGen MDEP VCEP but has a greater Grantham distance than p.Gly261Glu​(PM5_Supporting). In summary, this variant meets the criteria to be classified as pathogenic for monogenic diabetes. ACMG/AMP criteria applied, as specified by the ClinGen MDEP VCEP (specification version 1.3.0, approved 8/11/2023): PM2_Supporting, PP2, PP3, PM5_Supporting, PS4, PP1_Strong, PS3_Moderate.