The c.1055T>G variant in the glucokinase gene, GCK, causes an amino acid change of leucine to arginine at codon 352 (p.(Leu352Arg)) 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 absent from gnomAD v4.1.0 genomes (PM2_Supporting). This variant is predicted to be deleterious by computational evidence, with a REVEL score of 0.994, 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-hyperglycemia (fasting glucose 5.5-8 mmol/L and HbA1c 5.6 - 7.6% and a 3 generation dominant family history of hyperglycemia) (PP4_Moderate; PMID: 24804978). This variant segregated with diabetes with two informative meioses in a single family; however, this does not meet the thresholds for PP1 set by the ClinGen MDEP (PMID: 27236918, 24804978). Another missense variant at the same residue, c.1055T>C (p.Leu352Pro), has been classified as likely pathogenic by the ClinGen MDEP (PM5_Supporting). In summary, c.1055T>G meets the criteria to be classified as likely pathogenic for monogenic diabetes. ACMG/AMP criteria applied, as specified by the ClinGen MDEP (specification version 2.0.0, approved 2/17/2025): PP2, PP3, PM2_Supporting, PP4_Moderate, PM5_Supporting.