OTC and hemophilia B: Gene therapy has been successfully applied in preclinical models for IEMs, including glycogen storage disease type 1a (GSD Ia; OMIM #232200), familial hypercholesterolemia (OMIM #143890), hemophilia B (OMIM #306900), ornithine transcarbamylase (OTC; EC 2.1.3.3) deficiency (OMIM #311250), hereditary tyrosinemia type 1 (HT1; OMIM #276700), and alpha‐1 antitrypsin deficiency (OMIM #613490).2, 3, 4 As a consequence of these successes, various gene therapy trials are currently ongoing.