The c.1113C>G variant in the glucokinase gene, GCK, causes an amino acid change of cysteine to tryptophan at codon 371 (p.(Cys371Trp)) of NM_000162.5. GCK is defined by the ClinGen MDEP VCEP as a gene that has a low rate of benign missense variation and where pathogenic missense variants are a common mechanism of disease (PP2). This variant is predicted to be deleterious by computational evidence, with a REVEL score of 0.891, which is greater than the MDEP VCEP threshold of 0.70 (PP3). This variant is absent from gnomAD v2.1.1 (PM2_Supporting). 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 OGTT increment < 3 mmol/L and a 3-generation family history of diabetes) (PP4_Moderate; internal lab contributors). This variant was identified in three unrelated individuals with a clinical picture consistent with monogenic diabetes; however, PS4_Moderate cannot be applied because this number is below the MDEP threshold (internal lab contributors). This variant segregated with diabetes with one informative meiosis in a single family; however, this does not meet the thresholds for PP1 set by the ClinGen MDEP (PMID: 27236918; internal lab contributors). Another missense variant, c.1112G>T p.(Cys371Phe), has been interpreted as pathogenic by the ClinGen MDEP VCEP and p.Cys371Trp has a greater Grantham distance. (PM5). In summary, this 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 1.2.0 approved 6/7/2023): PP2, PP3, PM2_Supporting, PP4_Moderate, PM5 .