The c.1160C>T variant in the glucokinase gene, GCK, causes an amino acid change of alanine to valine at codon 387 (p. (Ala387Val)) of NM_000162.5. This variant has an incomputable gnomAD v2.1.1 Popmax filtering allele frequency due to no copies in the European non-Finnish subpopulation and one copy 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). 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 located in the larger hexokinase domain of the GCK gene (PMID: 31638168) but this variant does not reside in an amino acid that directly binds glucose or ATP, which is defined as critical for the protein’s function by the ClinGen MDEP, so it does not meet PM1. This variant is predicted to be deleterious by computational evidence, with a REVEL score of 0.871 which is greater than the MDEP VCEP threshold of 0.70 (PP3). Another missense variant, c.1159G>A p.(Ala387Thr) has been interpreted as pathogenic by the ClinGen MDEP, and p.Ala387Glu has a greater Grantham distance (PM5). This variant was identified in four unrelated individuals with hyperglycemia (PS4_Moderate; PMID: 34421822, Internal lab contributors). 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 negative antibodies) (PP4_Moderate; PMID: 34421822). This variant segregated with hyperglycemia, with five informative meioses in three families (PP1_Strong; PMID: 34421822, Internal lab contributor). In summary, c.1160C>T meets the criteria to be classified as pathogenic for monogenic diabetes. ACMG/AMP criteria applied, as specified by the ClinGen MDEP VCEP (VCEP specifications version v1.3.0; approved 8/11/2023): PP2, PP3, PM2_Supporting, PP1_Strong, PM5, PS4_Moderate, PP4_Moderate.