The c.605T>G variant in the glucokinase gene, GCK, causes an amino acid change of methionine to arginine at codon 202 (p.(Met202Arg)) 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 0.969, which is greater than the MDEP VCEP threshold of 0.70 (PP3). This variant is absent from gnomAD v2.1.1 (PM2_Supporting). This variant was identified in 7 unrelated individuals with hyperglycemia (PS4; internal lab contributors). This variant segregated with diabetes, with 3 informative meioses in 2 families with MODY (PP1_Moderate; 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 three-generation, dominant family history of diabetes or hyperglycemia in a family not used for PP1) (PP4_Moderate; internal lab contributors). Another missense variant, c.605T>C p.Met202Thr has been interpreted as pathogenic by the ClinGen MDEP, and p.Met202Arg has an equal or greater Grantham distance (PM5). In summary, c.605T>G 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): PM5, PS4, PP4_Moderate, PP1_Moderate, PM2_Supporting, PP2, PP3.