The c.509_517dup variant in the glucokinase gene, GCK, is a 9 base pair insertion resulting in the in-frame addition of 3 amino acid(s) at codon 173 (p.(Lys172_Ala173insGlyPheLys)) within exon 5 of NM_000162.5. The c.509_517dup variant is predicted to change the length of the protein due an in-frame insertion of 3 amino acids in a nonrepeat region (PM4). This variant is absent in gnomAD v2.1.1 (PM2_Supporting), was identified in two unrelated individuals with mildly elevated HbA1c; however, PS4_Moderate cannot be applied because this number is below the ClinGen MDEP threshold (Internal contributors). One of these individuals has a phenotype specific for GCK-MODY (persistent FBG 5.5-8 mmol/L and HbA1c 5.6 - 7.6%) (PP4_Moderate; Internal contributor). The Relative Activity Index (RAI) of this variant was found to be 0, which is below the MDEP cutoff (<0.5) (PS3_Moderate; Internal lab contributor). In summary, the c.509_517dup variant meets the criteria to be classified as likely pathogenic for monogenic diabetes. ACMG/AMP criteria applied, as specified by the ClinGen MDEP (specification version 1.1, approved 3/23/23): PM4, PM2_Supporting, PP4_Moderate, PS3_Moderate.