Physician-scientist Katherine High, M.D., cofounder, president, and head of R&D at Spark Therapeutics, has had to solve many technological and clinical problems in her 30-year-long quest to turn genes into medicines for patients with hemophilia and other diseases caused by a single flawed or missing gene. For example, there was the problem that halted the second clinical trial of an experimental gene therapy for hemophilia B. The therapy was designed to deliver a functional copy of the gene for the blood-clotting factor to a patient’s liver cells. The Phase 1 trial, initiated in 2001, was sponsored by the now-defunct biotech company Avigen. Dr. High, who had collaborated with Avigen scientists in the design of the gene therapy’s recombinant adeno-associated viral (AAV) vector, was then head of hematology research at Children’s Hospital of Philadelphia (CHOP).