BioMarin Pharmaceutical is developing pegvaliase (PALYNZIQ™) as a treatment for phenylketonuria, a genetic disorder caused by deficiency of phenylalanine hydroxylase which leads to neurotoxic accumulation of phenylalanine. Data from the phase III PRISM clinical trial program indicate treatment with pegvaliase is associated with sustained reductions in blood phenylalanine levels and sustained improvements in neurological sequelae in patients with phenylketonuria. Based on these positive results pegvaliase was recently approved in the US for adults with phenylketonuria who have uncontrolled blood phenylalanine concentrations on current treatment. This article summarizes the milestones in the development of pegvaliase leading to this first approval.