The c.666_668del variant in the HNF1 homeoboxA gene, HNF1A, results in a deletion of Lysine at codon 222 of NM_000545.6 (PM4_Supporting). Functional in vitro studies demonstrated that cells with this variant showed severely impaired transactivation activity and DNA binding (significantly below 40%). Furthermore, cells with this variant showed low protein expression levels and impaired nuclear targeting with elevated immunofluorescence signal in the cytoplasm compared to WT HNF1A (PS3; Janne Molnes, personal communication). 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 two individuals with a clinical history highly specific for HNF1A-MODY (MODY probability calculator result >50%, negative genetic testing for HNF4A, and sulfonylurea-responsiveness in both and negative antibodies in one) (PP4_Moderate; PMID: 27913849, internal lab contributor). This variant segregated with disease with three informative meioses in a single family with MODY (PP1; internal lab contributor). Additionally, this variant is absent from gnomAD v2.1.1 (PM2_Supporting). In summary, c.666_668del meets the criteria to be classified as likely pathogenic for monogenic diabetes. ACMG/AMP criteria applied, as specified by the ClinGen MDEP VCEP (specification version 1.1, approved 9/30/2021): PS3, PM1, PP4_Moderate, PM2_Supporting, PM4_Supporting, PP1.