EN1 and Parkinson disease: Recombinant En1/2 proteins can rescue mDA neuronal loss and increase physiological activities of mDA neurons in several PD models, including mutated En1 or alpha-synuclein mice, and chemically induced PD models [e.g., MPTP, rotenone, 6-OHDA (6-hydroxydopamine)] (Pen et al., 2008; Alvarez-Fischer et al., 2011).