The c.389T>CT variant in the glucokinase gene, GCK, causes an amino acid change of isoleucine to threonine at codon 130 (p.(Ile130Thr)) 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 0.932, which is greater than the MDEP VCEP threshold of 0.70 (PP3). MDEP wild type quality control measures were met, and the relative activity Index (RAI) of this variant was found to be 0.17, which is below the MDEP cutoff (<0.5) (PMID: 22291974). This variant has an incomputable gnomAD v2.1.1 Popmax filtering allele frequency due to 1 copy in the European non-Finnish subpopulation and 0 copies in any other subpopulation, thereby meeting the ClinGen MDEP threshold criteria for PM2_Supporting (ENF Popmax FAF <= 0.000003 and <= 2 copies in ENF and <=1 copy in any other subpopulation) (PM2_Supporting). This variant was identified in at least 6 unrelated individuals with hyperglycemia (PS4_Moderate; PMIDs: 36257325, 19790256, 14517956, 22291974, 34362814, internal lab contributor). One of these individuals had 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) (PP4_Moderate; PMID: 22291974). In summary, c.389T>C meets the criteria to be classified as likely pathogenic for monogenic diabetes. ACMG/AMP criteria applied, as specified by the ClinGen MDEP (specification version 1.3, approved 8/11/2023): PP4_Moderate, PS3_Moderate, PS4_Moderate, PP2, PP3, PM2_Supporting.