The c.781G>A variant in the glucokinase gene, GCK, causes an amino acid change of glycine to arginine at codon 261 (p.(Gly261Arg)) 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 0.914, which is greater than the MDEP threshold of 0.70 (PP3). This variant has an incomputable gnomAD v2.1.1 Popmax minor filtering allele frequency due to 0 copies in the European non-Finnish subpopulation and 1 copy in the "Other" subpopulation, thereby meeting the ClinGen MDEP threshold criteria for PM2_Supporting (ENF Popmax FAF less than or equal to 0.000003 and <= 2 copies in ENF and <=1 copy in any other subpopulation) (PM2_Supporting). This variant was identified in more than 38 unrelated individuals with diabetes (PS4; PMIDs: 29417725, 33324081, 21518409, 25015100, 22611063, 17573900, internal lab contributors). This variant was identified in at least two individuals with a clinical history highly specific for GCK-hyperglycemia (FBG 5.5-8 mmol/L and HbA1c 5.6 - 7.6% and negative antibodies) (PP4_Moderate, internal lab contributors). This variant segregated with diabetes/hyperglycemia with 30 informative meioses in 18 families (PP1_Strong; PMID 33324081, 21518409, internal lab contributors). This variant was identified as a de novo occurrence with confirmed parental relationships in one individual with a clinical picture consistent with GCK-hyperglycemia (PS2; internal lab contributors). This variant has been detected in at least 2 individuals with neonatal diabetes who were both homozygous for the variant, confirmed in trans (PM3: PMID 33324081, 25015100, 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.Gly261Arg variant has a relative activity index (RAI) of 0.02, which is less than the MDEP VCEP threshold of 0.50 (PMID: 19903754). In summary, this variant meets the criteria to be classified as pathogenic for monogenic diabetews. ACMG/AMP criteria applied, as specified by the ClinGen MDEP VCEP (specification version 1.3.0, approved 8/11/2023): PM2_Supporting, PP2, PP3, PP4_Moderate, PS3_Moderate, PM3, PP1_Strong, PS4, PS2.