The c.748C>T variant in the glucokinase gene, GCK, causes an amino acid change of arginine to cysteine at codon 250 (p.(Arg250Cys)) of NM_000162.5. This variant is absent from gnomAD v2.1.1 (PM2_Supporting). GCK: This variant was identified in 11 unrelated individuals with hyperglycemia (PS4; PMIDs: 30245511, 17204055, 19069349, 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 OGTT increment < 3 mmol/L and 3 generation family history of diabetes/hyperglycemia (PP4_Moderate; internal lab contributors). This variant segregated with diabetes/hyperglycemia, with 5 informative meioses in 2 families (PP1_Strong; PMID: 30245511, internal lab contributors). 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 0.944 which is greater than the MDEP VCEP threshold of 0.70 (PP3). In summary, c.748C>T 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): PM2_Supporting, PS4, PP4_Moderate, PP1_Strong, PP2, PP3.