In February, Synlogic cut 90 percent of its workforce as it ended a pivotal study of its lead experimental therapy to treat the rare, metabolic condition phenylketonuria. The decision came in response to results of an internal review that indicated the trial was unlikely to meet its primary endpoint. Synlogic, which had been developing a new class of therapies using synthetic biology, is now weighing its strategic options. We spoke to Neal Sondheimer, outgoing head of clinical for Synlogic and adjunct associate professor of pediatrics and molecular genetics for The Hospital for Sick Children in Toronto, about PKU, the treatment options for people with the condition, and the consequences of the surprising results from the Synlogic study.