The c.1016A>G variant in the glucokinase gene, GCK, causes an amino acid change of glutamic acid to glycine at codon 339 (p.(Glu339Gly)) 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 0.993, which is greater than the MDEP VCEP threshold of 0.70 (PP3). This variant is absent from gnomAD v2.1.1 (PM2_Supporting). This variant was identified in 8 unrelated individuals with hyperglycemia (PS4; ClinVar: 393450, PMIDs: 18399931, 16731834, internal lab contributors). This variant segregated with hyperglycemia, with 3 informative meioses in 1 family (PP1; internal lab contributors). MDEP wild type quality control measures were met, and the relative activity Index (RAI) of this variant was found to be 0.06, which is below the MDEP cutoff (<0.5) (PS3_Moderate; PMID: 16731834). Another missense variant, c.1015G>A p.Glu339Lys, has been classified as likely pathogenic by the ClinGen MDEP (PM5_Supporting). In summary, c.1016A>G 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): PS4, PS3_Moderate, PP2, PP3, PM2_Supporting, PM5_Supporting, PP1.