The c.764C>T variant in the glucokinase gene, GCK, causes an amino acid change of threonine to isoleucine at codon 255 (p.(Thr255Ile)) of transcript 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 0.795, which is greater than the MDEP VCEP threshold of 0.70 (PP3). This variant resides in an amino acid that directly binds glucose, which is defined as critical for the protein’s function by the ClinGen MDEP (PM1). This variant is absent from gnomAD v4.1.0 (PM2_Supporting). This variant was identified in five unrelated individuals with hyperglycemia (PS4_Moderate, PMID: 28170077; 19410318; 35472491). Additionally, at least two of these individuals had a linical history highly specific for GCK-hyperglycemia (fasting glucose 5.5-8 mmol/L, HbA1c 5.6 - 7.6% and detectable C-peptide in pediatric patients after 3 years) (PP4_Moderate; PMID: 28170077). This variant was also detected in the homozygous state in one individual with neonatal diabetes (PM3_Supporting; PMID: 36510364). Other missense variants at the same residue, c.764C>G (p.Thr255Ser) and c.763A>G (p.Thr255Ala), have been interpreted as pathogenic by the ClinGen MDEP, and p.Thr255Ile has a greater Grantham distance (PM5_Strong). In summary, c.764C>T meets the criteria to be classified as pathogenic for monogenic diabetes. ACMG/AMP criteria applied, as specified by the ClinGen MDEP (specification version 3.1.0, approved 10/10/2025): PM1, PM2_Supporting, PM3_Supporting, PM5_Strong, PP2, PP3, PP4_Moderate, PS4_Moderate.