The c.130delC variant in the HNF1 homeobox A gene, HNF1A, causes a frameshift in the protein at codon 44 (NM_000545.8), adding 111 novel amino acids before encountering a stop codon (p.(Leu44TrpfsTer111)). This variant, located in biologically-relevant exon 1 of 10, is predicted to lead to nonsense mediated decay in a gene in which loss-of-function is an established disease mechanism (PVS1; PMID: 23348805) and is absent from gnomAD v2.1.1 (PM2_Supporting). Additionally, this variant was identified in an individual with a clinical history highly specific for HNF1A-MODY (MODY probability calculator result >50%, negative genetic testing for HNF4A) (PP4; PMID: 15305805). This variant segregated with diabetes, with at least five informative meioses in two families with MODY (PP1_Strong; PMIDs: 15305805 and 12453976). 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 (PMIDs: 15305805 and 12453976). In summary, c.130delC meets the criteria to be classified as pathogenic for monogenic diabetes. ACMG/AMP criteria applied, as specified by the ClinGen MDEP (specification version 1.1, approved 9/30/21): PVS1, PM2_Supporting, PP1_Strong, PP4.