The c.788G>T variant in the HNF1 homeobox A gene, HNF1A, causes an amino acid change of arginine to leucine at codon 263 (p. (Arg263Leu) of transcript, e.g. NM_000545.8. This variant resides in an amino acid within the HNF1α DNA binding domain that directly binds DNA, which is defined as critical for the protein’s function by the ClinGen MDEP (PM1). Additionally, this variant is predicted to be deleterious by computational evidence, with a REVEL score of 0.933, which is greater than the MDEP VCEP threshold of 0.70 (PP3) and is absent from gnomAD v2.1.1(PM2_Supporting). Two other missense variants, c.787C>T (p.Arg263Cys) and c.788G>A (p.Arg263His), have been interpreted as pathogenic by the ClinGen MDEP, and p.Arg263Leu has a greater Grantham distance than p.Arg263His (PM5_Strong). Functional studies demonstrated the p.Arg263Leu protein has no DNA binding activity, indicating that this variant impacts protein function (PS3_Supporting, PMID: 12712243). This variant segregated with diabetes, with three informative meioses in one family with MODY (PP1; PMID: 12712243). This variant was identified in an individual with a clinical history suggestive of HNF4A-MODY (MODY probability calculator result >50%); however, HNF1A was not tested, and PP4 is not applied (PMID: 12712243). In summary, c.790G>T 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 9/30/2021): PM1, PM2_Supporting, PP3, PM5_Strong, PS3_Supporting, PP1.