The c.171G>T variant in the glucokinase gene, GCK, causes an amino acid change of methionine to isoleucine at codon 57 (p.(Met57Ile)) of NM_000162.5. This variant is absent from gnomAD v2.1.1 and v4.1 (PM2_Supporting). 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 0.987, which is greater than the MDEP VCEP threshold of 0.70 (PP3). This variant was identified in an individual with a clinical history highly specific for GCK-MODY (FBG 5.5-8 mmol/L and HbA1c 5.6 - 7.6%) (PP4; PMID: 34440516). The nucleotide change c.171G>A, which results in the same amino acid change, has been classified as pathogenic for GCK-hyperglycemia by the ClinGen MDEP (PS1). Another missense variant at the same codon, c.170T>G (p.Met57Arg), has been classified as likely pathogenic by the ClinGen MDEP (PM5_Supporting). In summary, c.171G>T p.(Met57Ile) meets the criteria to be classified as likely pathogenic for monogenic diabetes. ACMG/AMP criteria applied, as specified by the ClinGen MDEP (specification version 1.3.0, approved 8/11/2023): PM2_Supporting, PM5_Supporting, PP2, PP3, PP4, PS1).