The c.127C>T variant in the glucokinase gene, GCK, causes an amino acid change of arginine to cysteine at codon 43 (p.(Arg43Cys)) 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 0.924, which is greater than the MDEP VCEP threshold of 0.70 (PP3). Additionally, established functional studies demonstrated the p.Arg43Cys protein has a relative activity index (RAI) < 0.5, indicating that this variant impacts protein function (PS3_Moderate; PMID: 25015100). This variant is absent in gnomAD v2.1.1 (PM2_Supporting), and was identified in at least eight unrelated individuals with hyperglycemia/diabetes (PS4; PMIDs: 30592380, 21348868, PMID 23771172, 25015100). One of these individuals was homozygous for c.127C>T and had permanent neonatal diabetes mellitus (PNDM) and negative testing for ABCC8, KCNJ11, INS and EIF2AK3 (PP4; PMID 25015100). This variant also segregated with diabetes/hyperglycemia, with 5 informative meioses in one family (PP1_Strong; PMID 21348868). Another missense variant, c.128G>A (p.Arg43His), has been interpreted as pathogenic by the ClinGen MDEP and p.Arg43Cys has a greater Grantham distance (PM5). Taken together, this evidence supports the classification of this variant as pathogenic for monogenic diabetes. ACMG/AMP criteria applied, as specified by the ClinGen MDEP (specification version 1.3.0, approved 8/11/2023): PP1_Strong, PS4, PM5, PP2, PP3, PP4, PM2_Supporting, PS3_Moderate.