The c.1del variant in the hepatic nuclear factor 4-alpha gene, HNF4A, results in the loss of the initiation codon (p.Met1?) of NM_175914.5. By altering the start codon of the coding sequence, this variant may cause a truncated or absent protein in a gene in which loss-of-function is an established disease mechanism (PMID: 23348805).This variant is absent from gnomAD v2.1.1 (PM2_Supporting). This variant was identified in an individual with a clinical history highly specific for HNF4A-MODY (MODY probability calculator result >50%, negative genetic testing for HNF1A, and sulfonylurea responsive) (PP4_Moderate; Internal lab contributor). This variant segregated with diabetes, with three informative meioses in one family (PP1; Internal lab contributors). In summary, c.1del meets the criteria to be classified as likely pathogenic for monogenic diabetes. ACMG/AMP criteria applied, as specified by the ClinGen MDEP (specification version 2.0.0, approved 10/11/1024): PVS1_Strong, PP4_Moderate, PP1, PM2_Supporting.