The c.1391_1392delGC variant in the glucokinase gene, GCK, causes a frameshift in the protein at codon 464 (NM_000162.5), adding 69 novel amino acids before encountering a stop codon (p.(p.Gly464AlafsTer69)). 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). This variant is absent from gnomAD v2.1.1 (PM2_Supporting). The variant was reported in an individual with MODY, autosomal dominant family history of diabetes, antibody negativity, non-insulin dependence, and segregation with diabetes, but there was insufficient information given to apply PP4 or PP1 (PMID: 29056535). In summary, c.1391_1392delGC meets the criteria to be classified as likely pathogenic for monogenic diabetes. ACMG/AMP criteria applied, as specified by the ClinGen MDEP (specification version 1.1.0, approved 3/23/2023): PVS1, PM2_Supporting.