ADA and metabolic disease: It wouldn’t be until 1990—nearly 36 years from the publication of Jatzkewitz’s initial work—that the first polymer-drug conjugate would receive market approval in the form of Adagen, adenosine deaminase protein conjugated with 5 kDa poly (ethylene glycol), or PEG, used to treat a rare and hereditary, pediatric metabolic disorder called adenosine deaminase severe combined immunodeficiency (Hershfield et al., 1987).