The c.503C>T variant in the glucokinase gene, GCK, causes an amino acid change of threonine to isoleucine at codon 168 (p.(Thr168Ile)) of NM_000162.5. This variant is absent from gnomAD v2.1.1 (PM2_Supporting). 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 0.997 which is greater than the MDEP VCEP threshold of 0.70 (PP3). 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 was identified in an individual with a clinical history highly specific for GCK-hyperglycemia (FBG 5.5-8 mmol/L and HbA1c 5.6 - 7.6%) (PP4; internal lab contributors). This variant was identified in 4 unrelated individuals with hyperglycemia (PS4_Moderate; internal lab contributors). Two other missense variants, c.502A>G p.Thr168Ala and c.503C>A p.Thr168Asn, have been interpreted as pathogenic by the ClinGen MDEP, and p.Thr168Ile has a greater Grantham distance than p.Thr168Ala and p.Thr168Asn (PM5_Strong). In summary, c.503C>T 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): PM2_Supporting, PP2, PP3, PM1, PP4, PS4_Moderate, PM5_Strong.