The c.911T>C variant in the glucokinase gene, GCK, causes an amino acid change of leucine to proline at codon 304 (p.(Leu304Pro)) 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 0.874, which is greater than the MDEP threshold of 0.70 (PP3). This variant has an incomputable gnomAD v2.1.1 Popmax minor filtering allele frequency due to 1 copy in the European non-Finnish subpopulation and 0 copies in any other 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). This variant was identified in 13 unrelated individuals with hyperglycemia (PS4; PMID 29207974, 11942313, internal lab contributors). This variant segregated with diabetes with 4 informative meioses in 3 families with diabetes/hyperglycemia (PP1_Strong; 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%) (PP4; internal lab contributors). 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.Leu304Pro variant has RAI=0.19, which is less than the MDEP VCEP threshold of 0.50. (PMIDs: 29704611, 26208450). 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 1.2.0, approved 6/7/2023): PM2_Supporting, PP3, PP2, PP4, PS4, PP1_Strong, PS3_Supporting.