The c.675del variant in the HNF1 homeobox A gene, HNF1A, causes a frameshift in the protein at codon 225 (NM_000545.8), adding 8 novel amino acids before encountering a stop codon (p.(Ser225ArgfsTer8)). This variant, located in biologically-relevant exon 3 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 is absent in gnomAD v2.1.1 (PM2_Supporting). This variant was identified in at least 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:11806470, PMID:33245425, PMID:1883825). One of the individuals and her sister had a phenotype suggestive of HNF1A-monogenic diabetes; however, the MODY probability was slightly below 50% and HNF4A was not evaluated, and PP4 could not be applied (PMID:11806470). This variant segregated with diabetes, with at least seven informative meioses in two families with MODY (PP1; PMID:11806470, PMID:33245425). In summary, c.675del meets the criteria to be classified as pathogenic for monogenic diabetes. ACMG/AMP criteria applied, as specified by the ClinGen MDEP (specification version 2.1.0, approved 8/11/2023): PVS1, PP1_Strong, PM2_Supporting.