The c.676A>G variant in the HNF1 homeobox A gene, HNF1A, causes an amino acid change of lysine to glutamic acid at codon 226 (p.(Lys226Glu)) of NM_000545.8. This variant is located within a conserved region of the DNA binding domain (codons 107-174 and 201-280) of HNF1A, which is defined as critical for the protein’s function by the ClinGen MDEP (PM1_Supporting). This variant is predicted to be deleterious by computational evidence, with a REVEL score of 0.815, which is greater than the MDEP VCEP threshold of 0.70 (PP3). This variant is absent from gnomAD v2.1.1 (PM2_Supporting), and 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:18003757, internal lab contributors). One individual has a clinical history highly specific for HNF1A-MODY (MODY probability calculator result >50%, negative genetic testing for HNF4A, and negative antibodies) (PP4_Moderate; internal lab contributor). This variant segregated with diabetes, with at least four informative meioses in two families with MODY (PP1_Strong; internal lab contributors). In summary, c.676A>G 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 8/11/2023): PP1_Strong, PP4_Moderate, PP3, PM1_Supporting, PM2_Supporting.