The c.676G>A variant in the glucokinase gene, GCK, causes an amino acid change of valine to methionine at codon 261 (p.(Val226Met)) 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 0.789, which is greater than the MDEP threshold of 0.70 (PP3). This variant has a gnomAD v2.1.1 Popmax filtering allele frequency of 0.000002920 (below the MDEP threshold of 0.000003) and ≤ 2 copies observed in the European non-Finnish population and ≤ 1 copy in any other subpopulation, thereby meeting the ClinGen MDEP criteria for PM2_Supporting (PM2_Supporting). This variant was identified in more than 70 unrelated individuals with hyperglycemia (PS4; internal lab contributors). 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% and negative antibodies) (PP4_Moderate; internal lab contributors). This variant segregated with diabetes/hyperglycemia with more than 20 informative meioses in more than 30 families (PP1_Strong; internal lab contributors). 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). A kinetic analysis of recombinant wild-type (WT) and mutant glucokinase demonstrated that the wild-type kinetic parameters pass the quality control, the wild-type ATP Km is between 0.4-0.65, and the p.Val226Met variant has a relative activity index (RAI) of 0.009 to 0.198, which is less than the MDEP VCEP threshold of 0.50 (PS3_Moderate; PMID: 10525657, 25015100, https://doi.org/10.1159/isbn.978-3-318-01080-0​). In summary, this variant meets the criteria to be classified as pathogenic for monogenic diabetes. ACMG/AMP criteria applied, as specified by the ClinGen MDEP VCEP (specification version 1.3.0, approved 8/11/2023): PS4, PP1_Strong, PS3_Moderate, PM2_Supporting, PP2, PP4_Moderate, PP3, PM1.