The c.764C>G variant in the glucokinase gene, GCK, causes an amino acid change of threonine to serine at codon 255 (p.(Thr255Ser)) of transcript NM_000162.5. This variant resides in an amino acid that directly binds glucose, which is defined as critical for the protein’s function by the ClinGen MDEP (PM1). This variant is absent from gnomAD v4.1.0 (PM2_Supporting). This variant segregated with hyperglycemia, with six informative meioses in four families (PP1_Strong; Internal lab contributors). This variant was identified in multiple individuals with a clinical history highly specific for GCK-hyperglycemia (fasting glucose 5.5-8 mmol/L and HbA1c 5.6 - 7.6% and negative antibodies) (PP4_Moderate; internal lab contributors). This variant was identified in five unrelated individuals with hyperglycemia (PS4_Moderate; PMID 24808978, 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 0.714, which is greater than the MDEP VCEP threshold of 0.70 (PP3). In summary, c.764C>G meets the criteria to be classified as pathogenic for monogenic diabetes. ACMG/AMP criteria applied, as specified by the ClinGen MDEP (specification version 3.1.0, approved 10/10/2025): PM1, PM2_Supporting, PP2, PP3, PP4_Moderate, PS4_Moderate, PP1_Strong