The c.179C>T variant in the glucokinase gene, GCK causes an amino acid change of threonine to isoleucine at codon 60 (p.(Thr60Ile)) 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 0.979, which is greater than the MDEP VCEP threshold of 0.70 (PP3). This variant has an incomputable gnomAD v4.1.0 Grpmax filtering allele frequency due to a single copy present in the South Asian population and 0 copies in any other subpopulation thereby meeting the ClinGen MDEP threshold criteria for PM2_Supporting (gnomAD v4.1 Grpmax FAF ≤ 0.000003) (PM2_Supporting). This variant was identified in at least 11 unrelated individuals with hyperglycemia (PS4; PMID: 18248649, 34746319, internal lab contributors). At least one of these individuals had a clinical history highly specific for GCK-hyperglycemia (fasting glucose 5.5-8 mmol/L and HbA1c 5.6 - 7.6%) (PP4; internal lab contributors). Additionally, this variant segregated with hyperglycemia with eight meioses in six families (PP1_Strong; PMID: 34746319, internal lab contributors). Functional studies of this variant demonstrate that it disrupts normal GK-GKRP binding (PS3_Supporting; PMID: 33273586). In summary, c.179C>T meets the criteria to be classified as pathogenic for monogenic diabetes. ACMG/AMP criteria applied, as specified by the ClinGen MDEP (specification version 2.0.0, approved 2/17/2025): PS4, PP1_Strong, PP2, PP3, PP4 PM2_Supporting, PS3_Supporting.