The c.823C>T variant in the glucokinase gene, GCK, causes an amino acid change of arginine to cysteine at codon 275 (p.(Arg275Cys)) of NM_000162.5. This variant was identified in 7 unrelated individuals with hyperglycemia (PS4; PMID: 24001579, internal lab contributors). This variant was identified in two individuals with 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) (internal lab contributors). This variant segregated with diabetes/hyperglycemia, with 3 informative meioses in 1 family (PP1; PMID: 24001579). 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 0.901, which is greater than the MDEP VCEP threshold of 0.70 (PP3). While a study exploring the effect of this variant on protein function has been performed and suggests a possible impact on stability and dimerization, the study does not meet the criteria set forth by the MDEP for the application of PS3 or BS3 (PMID: 24001579). This variant has an incomputable gnomAD v2.1.1 Popmax minor filtering allele frequency due to 1 copy in the European non-Finnish subpopulation, 1 copy in the African/African American subpopulation, and 1 copy in the South Asian subpopulation, thereby meeting the ClinGen MDEP threshold criteria for PM2_Supporting (ENF Popmax FAF less than or equal to 0.000003 and <= 2 copies in ENF and <=1 copy in any other subpopulation) (PM2_Supporting). In summary, 823C>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): PS4, PP4_Moderate, PP1, PP2, PP3, PM2_Supporting.