The c.820G>T variant in the glucokinase gene, GCK, is a missense variant resulting in an amino acid change of aspartic acid to tyrosine at codon 274 (p.(Asp274Tyr)) 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). Additionally, it is predicted to be deleterious by computational evidence, with a REVEL score of 0.99, which is greater than the MDEP VCEP threshold of 0.70 (PP3). This variant segregated with hyperglycemia with four informative meioses in a single family (PP1_Moderate, internal lab contributors). This variant was identified in an individual with a clinical history highly specific for GCK-hyperglycemia (fasting glucose 5.5-8 mmol/L and HbA1c 5.6 - 7.6% and negative antibodies). (PP4_Moderate; internal lab contributors). Another missense variant at the same amino acid, c.820G>A p.(Asp274Ala), has been classified as pathogenic by the ClinGen MDEP and p.Asp274Tyr has a larger Grantham distance (PM5). In summary, c.820G>T variant meets the criteria to be classified as likely pathogenic for monogenic diabetes. ACMG/AMP criteria applied, as specified by the ClinGen MDEP VCEP (specification version 1.3.0, approved 8/11/2023): PP2, PP3, PP1_Moderate, PP4_Moderate, PM2_Supporting, PM5.