The c.864del variant in the HNF1 homeobox A gene, HNF1A, causes a frameshift in the protein at codon 291 (NM_000545.8), adding 51 novel amino acids before encountering a stop codon (p.Pro291GlnfsTer51). This variant, located in biologically-relevant exon 4 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). This variant was also identified in an individual with a clinical history highly specific for HNF1A-MODY (MODY probability calculator result >50%, negative genetic testing for HNF4A, and responded to low dose sulfonylureas) (PP4_Moderate; internal lab contributors). This variant failed QC in gnomAD and the frequency data is considered unreliable, therefore PM2_Supporting will not be applied. While this variant was identified in at least 7 unrelated individuals with non- autoimmune and non-absolute/near-absolute insulin-deficient diabetes, PS4_Moderate cannot be applied because the variant does not meet the PM2_Supporting (PMID: 26287533, internal lab contributors). In summary, c.864del meets the criteria to be classified as likely pathogenic for monogenic diabetes. ACMG/AMP criteria applied, as specified by the ClinGen MDEP (specification version 2.1.0, approved 8/11/23): PVS1, PP4_Moderate