The c.1335_1345del variant in the glucokinase gene, GCK, causes a frameshift in the protein at codon 447 (NM_000162.5), adding 8 novel amino acids before encountering a stop codon (p.(Arg447ProfsTer8)). While this variant, located in exon 10 of 10, is predicted to cause a premature stop codon and to escape nonsense mediated decay, it is in a functionally important region of a gene where loss-of-function is an established disease mechanism (PVS1; PMID: 19790256). Additionally, this variant is absent from gnomAD v2.1.1 (PM2_Supporting). This variant was identified in one individual with a diabetes; however this number does not meet the MDEP cutoff for PS4_Moderate. However, this individual has a clinical history highly specific for GCK-MODY (FBG 5.5-8 mmol/L and HbA1c 5.6 - 7.6%) (PP4; PMID: 14517956, internal lab contributor). This variant segregates with disease with 4 informative meiosis in this individual's family (PP1_moderate; internal lab contributor). In summary, the c.1335_1345del variant meets the criteria to be classified as pathogenic for monogenic diabetes. ACMG/AMP criteria applied, as specified by the ClinGen MDEP (specification version 1.2.0, approved 6/7/2023): PVS1, PP1_moderate, PP4, PM2_Supporting.