The c.1298_1309del variant in the glucokinase gene, GCK, is a 12 base pair deletion resulting in the in-frame deletion of 4 amino acid(s) at codon 433 (p.(Ser433_Ile436del)) within exon 10 of NM_000162.5. The c.1298_1309del variant is predicted to change the length of the protein due an in-frame deletion of 4 amino acids in a nonrepeat region (PM4). This variant is absent from gnomAD v2.1.1 (PM2_Supporting). This variant was identified in 3 unrelated individuals with hyperglycemia; however, PS4_Moderate cannot be applied because this number is below the ClinGen MDEP threshold (PMID: 12955723, internal lab contributors). One of these individuals had a clinical history highly specific for GCK-hyperglycemia (FBG 5.5-8 mmol/L and HbA1c 5.6 - 7.6% and negative antibodies) (PP4_Moderate; internal lab contributors). This variant segregated with hyperglycemia, with 2 informative meioses in 2 families (PP1; internal lab contributors). In summary, c.1298_1309del 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, approved 8/11/2023): PP4_Moderate, PM4, PP1, PM2_Supporting.