The authors report an adolescent who was found to have diabetes on routine blood testing. The initial diagnosis was type 2 diabetes because she was obese, did not have type 1 diabetes antibodies and both parents had diabetes. Highly sensitive C-reactive protein (hsCRP) was low in the proband and her father (≤0.1 mg/l) indicating that type 2 diabetes was unlikely, and that hepatocyte nuclear factor 1-α-maturity onset diabetes of the young (HNF1A-MODY) was the most likely diagnosis. Following a genetic diagnosis of HNF1A-MODY in the proband and her father, both patients were treated with gliclazide, with improvement in HbA1c. This case highlights the challenges of making a correct diagnosis of MODY in young onset diabetes. The authors report the first case where hsCRP, an easily available biomarker, has been used on an individual level to determine appropriate genetic testing of MODY in a family whose main differential diagnosis was familial type 2 diabetes.