The c.590T>A variant in the glucokinase gene, GCK, causes an amino acid change of methionine to lysine at codon 49 (p.(Met197Lys)) 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 absent from gnomAD v4.1.0 (PM2_Supporting). This variant was identified in three unrelated individuals with hyperglycemia; however, PS4_Moderate cannot be applied because this number is below the ClinGen MDEP threshold (PMID: 19790256; internal lab contributors). One of these individuals did have a clinical history highly specific for GCK-hyperglycemia (fasting glucose 5.5-8 mmol/L and HbA1c 5.6 - 7.6% and antibody negative) (PP4_Moderate; PMID: 19790256). This variant is predicted to be deleterious by computational evidence, with a REVEL score of 0.943, which is greater than the MDEP VCEP threshold of 0.70 (PP3). Additionally, functional studies meeting the MDEP wild type quality control measures showed that the relative activity index (RAI) of this variant was 0.02, which is below the MDEP cutoff (<0.5) (PS3_Moderate; PMID: 19336674). In summary, c.590T>A meets the criteria to be classified as likely pathogenic for monogenic diabetes. ACMG/AMP criteria applied, as specified by the ClinGen MDEP (specification version 3.1.0, approved 10/10/2025): PS3_Moderate, PP4_Moderate, PM2_Supporting, PP2, PP3.