The c.397G>T variant in the HNF1 homeobox A gene, HNF1A, causes an amino acid change of valine to leucine at codon 133 (p.(Val133Leu)) 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 also predicted to be deleterious by computational evidence, with a REVEL score of 0.947, which is greater than the MDEP VCEP threshold of 0.70 (PP3). This variant is absent in gnomAD v2.1.1 (PM2_Supporting), and was identified in an individual with a clinical history highly specific for HNF1A-MODY (MODY probability calculator result >50%, negative genetic testing for HNF4A) (PP4; internal lab contributors). This variant segregated with diabetes with two informative meioses in a single family; however, this does not meet the thresholds for PP1 set by the ClinGen MDEP (PMID: 27236918, internal lab contributors). Another missense variant, c.397G>A (p.Val133Met), has been classified as likely pathogenic by the ClinGen MDEP (PM5_Supporting). In summary, c.397G>T meets the criteria to be classified as a variant of uncertain significance for monogenic diabetes. ACMG/AMP criteria applied, as specified by the ClinGen MDEP (specification version 1.1, approved 9/30/21): PP3, PP4, PM1_Supporting, PM2_Supporting, PM5_Supporting.