The c.1305dup variant in the glucokinase gene, GCK, causes a frameshift in the protein at codon 436 (NM_000162.5), adding 23 novel amino acids before encountering a stop codon (p.(Ile436AspfsTer23)). 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). This variant is absent in gnomAD v2.1.1 (PM2_Supporting). This variant was identified in an individual with a clinical history consistent with GCK-MODY, but there was insufficient clinical information to evaluate for PP4. This variant was identified in two unrelated individuals with non-autoimmune and non-absolute/near-absolute insulin-deficient diabetes; however, PS4_Moderate cannot be applied because this number is below the ClinGen MDEP threshold (PMID: 31704690​, internal lab contributors). In summary, c.1305dup meets the criteria to be classified as likely pathogenic for monogenic diabetes. ACMG/AMP criteria applied, as specified by the ClinGen MDEP (specification version 1.3.0, approved 8/11/2023): PVS1, PM2_supporting.