The c.1307T>A variant in the glucokinase gene, GCK, causes an amino acid change of isoleucine to asparagine at codon 436 (p.(Ile436Asn)) 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 also predicted to be deleterious by computational evidence, with a REVEL score of 0.957, which is greater than the MDEP VCEP threshold of 0.70 (PP3). This variant is absent in gnomAD v2.1.1 (PM2_Supporting), and was identified in at least 5 unrelated individuals with hyperglycemia (PS4_Moderate; PMID 17204055, PMID 20337973, PMID 22493702, internal lab contributors). At least 2 of these individuals have a clinical history highly specific for GCK-hyperglycemia (FBG 5.5-8 mmol/L and HbA1c 5.6 - 7.6% and negative antibodies +/- 2 hour oral glucose tolerance test increment <3 mmol/L) (PP4_Moderate; internal lab contributors). This variant also segregated with diabetes/hyperglycemia, with 4 informative meioses in 2 families (PP1_Strong; internal lab contributors). Another missense variant, c.1306A>T p.Ile436Phe has been interpreted as likely pathogenic by the ClinGen MDEP (PM5_Supporting). While the RAI of this variant was 0.975, this variant results in decreased inhibition by GKRP (PS3_Supporting; PMID: 22493702). In summary, this evidence supports the classification of c.1307T>A 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_Moderate, PP4_Moderate, PM5_Supporting, PP2, PP3, PM2_Supporting.