The c.335C>T variant in the HNF1 homeobox A gene, HNF1A, causes an amino acid change of proline to leucine at codon 112 (p.(Pro112Leu)) of NM_000545.8. This variant is predicted to be deleterious by computational evidence, with a REVEL score of 0.966, which is greater than the MDEP VCEP threshold of 0.70 (PP3) and is absent from gnomAD v2.1.1 (PM2_Supporting). Additionally, 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 was identified in six unrelated individuals with non- autoimmune and non-absolute/near-absolute insulin-deficient diabetes (PS4_Moderate; internal lab contributors). This variant was identified in two individuals with a clinical history highly specific for HNF1A-MODY (MODY probability calculator result >50%, negative genetic testing for HNF4A, and response to low dose sulfonylureas) (PP4_Moderate; internal lab contributors). A luciferase assay meeting the ClinGen MDEP quality control specifications demonstrated that the p.Pro112Leu protein has transactivation activity below 40% of wildtype, indicating that this variant impacts protein function (PS3_Moderate, PMID: 32910913). Lastly, this variant segregated with diabetes, with 11 informative meioses in six families with MODY (PP1_Strong; internal lab contributors). In summary, c.335C>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 9/30/21): PP3, PM1_Supporting, PM2_Supporting, PP4_Moderate, PS4_Moderate,PS3_Moderate, PP1_Strong.