The c.1339del variant in the glucokinase gene, GCK, causes a frameshift in the protein at codon 447 (NM_000162.5), adding 167 novel amino acids before encountering a stop codon (p.(Arg447GlyfsTer167)). This variant, located in exon 10 of 10, is predicted to cause loss of a stop codon and result in an elongated protein. The additional residues are expected to cause improper folding, resulting in loss of function in a gene in which loss-of-function is an established disease mechanism (PVS1; PMID 19790256). This variant is absent from gnomAD v2.1.1 (PM2_Supporting). This variant was identified in 6 unrelated individuals with non-autoimmune/insulin-deficient diabetes (PMID 22761713; ClinVar ID 421604.2; internal lab contributors) (PS4_Moderate). This variant was identified in individuals with diabetes consistent with a GCK-MODY phenotype; however, PP4 is unable to be evaluated due to insufficient clinical information (PMID: 22761713, internal lab contributors). This variant segregated with disease with 2 informative meioses in a family with MODY, however this does not meet the thresholds for PP1 set by Jarvik and Browning (PMID: 27236918) (internal lab contributors). In summary, c.1339del meets the criteria to be classified as pathogenic for monogenic diabetes. ACMG/AMP criteria applied, as specified by the ClinGen MDEP (specification version 1.3.0, approved 8/11/2023): PVS1, PM2_Supporting, PS4_Moderate.