HNF1A and diabetes mellitus: Maturity onset diabetes of the young (MODY) is the most common form of monogenic diabetes, responsible for ∼3% of all diabetes diagnosed under 30 years old.1 It is an excellent exemplar for precision medicine as a genetic diagnosis can optimize treatment, for example, patients with HNF1A/HNF4A-MODY respond better to treatment by sulfonylurea tablets than insulin, whereas GCK-MODY does not need any treatment.2,3 Maturity onset diabetes of the young is classically thought of as autosomal dominant but that is now known to only capture part of the spectrum of adult-onset monogenic diabetes.