The c.925C>T variant in the Hepatocyte Nuclear Factor 4 Alpha gene, HNF4A, causes an amino acid change of Arginine to Cysteine at codon 309 (p.(Arg309Cys)) of NM_175914.5. This variant was identified in at least 10 unrelated individuals with non-autoimmune and non-absolute/near-absolute insulin-deficient diabetes (PS4; Internal lab contributors). This variant is located within the ligand-binding domain of HNF4A, which is defined as critical for the protein’s function by the ClinGen MDEP (PM1_Supporting). This variant has a gnomAD v2.1.1 Grpmax filtering allele frequency of 0.000002970 (below the MDEP threshold of 0.000003) and ≤ 2 copies observed in the European non-Finnish population and ≤ 1 copy in any other subpopulation, thereby meeting the ClinGen MDEP criteria for PM2_Supporting (PM2_Supporting). This variant segregated with diabetes, with 1 informative meiosis in each of 3 families with MODY (PP1_moderate; Internal lab contributors). This variant is predicted to be deleterious by computational evidence, with a REVEL score of 0.787, which is greater than to the MDEP VCEP threshold of 0.70 (PP3). This variant was identified in several individuals with a clinical history highly specific for HNF4A-MODY (MODY probability calculator result >50%, negative genetic testing for HNF1A, responsiveness to sulfonylureas, and 1 family member with persistent neonatal hypoglycemia) (PP4_Moderate; Internal lab contributors). In summary, c.925C>T meets the criteria to be classified as Pathogenic for monogenic diabetes. ACMG/AMP criteria applied, as specified by the ClinGen MDEP (specification version 2.0.0, approved 10/11/2023): PS4, PM1_Supporting, PM2_Supporting, PP1_Moderate, PP3, PP4_Moderate.