The c.1769-9_1770del variant in the HNF1 homeobox A gene, HNF1A, is predicted to remove a splice acceptor site in intron 9 of NM_000545.8. Given the predicted loss of the intron 9 acceptor site, this variant is predicted to result in the loss of exon 10 and to disrupt a significant part of the transactivation domain, a functionally important region of the protein. Even though this truncated transcript is predicted to escape nonsense mediated decay in a gene in which loss-of-function is an established mechanism of disease, there is clinical evidence that variants in this region lead to a monogenic diabetes phenotype. (PVS1_Strong; PMID: 23348805). This variant is absent from gnomAD v2.1.1 and v4.1 (PM2_Supporting). Additionally, this variant was identified in an individual with a clinical history highly specific for HNF1A-monogenic diabetes (MODY probability calculator result >50%, negative genetic testing for HNF4A, and sensitive to sulfonyureas) (PP4_Moderate; internal lab contributors). In summary, c.1769-9_1770del meets the criteria to be classified as likely pathogenic for monogenic diabetes. ACMG/AMP criteria applied, as specified by the ClinGen MDEP (specification version 2.1.1, approved 8/11/2023): PVS1_Strong, PM2_Supporting, PP4_Moderate).