The c.1169_1171del variant in the glucokinase gene, GCK, is a three-base pair deletion resulting in the in-frame deletion of isoleucine at codon 390 (p.(Ile390del)) within exon 9 of NM_000162.5. This variant is absent from gnomAD v2.1.1 (PM2_Supporting). The c.1169_1171del variant is predicted to change the length of the protein due to an in-frame deletion of a single amino acid in a non-repeat region (PM4_Supporting). This variant was identified in an individual with a clinical history highly specific for GCK-hyperglycemia (FBG 5.5-8 mmol/L and HbA1c 5.6 - 7.6% and OGTT with 2H OGTT increment <3 mmol/l) (PP4_Moderate; internal lab contributors). This variant segregated with hyperglycemia, with four informative meioses in one family (PP1_Moderate; internal lab contributors). This variant was identified in three unrelated individuals with hyperglycemia; however, PS4_Moderate cannot be applied because this number is below the ClinGen MDEP threshold (internal lab contributors). In summary, c.1169_1171del meets the criteria to be classified as likely pathogenic for monogenic diabetes. ACMG/AMP criteria applied, as specified by the ClinGen MDEP (specification version 1.3.0, approved 8/11/2023): PM2_supporting, PM4_supporting, PP4_moderate, PP1_moderate.