TH and Parkinson disease: Increased oxidative stress is considered to be one of the main factors involved in selective degeneration of DA neurons in aging and PD.3 Consistently, after dihydroethidium injection, we detected an increase in ethidium fluorescence in TH-positive neurons of DicerCKO mice 4 weeks after Tam treatment (Supplementary Figure S4D), indicative of elevated mitochondrial ROS production.35 These data show that similar to other animal models28 and PD patients,2 increased oxidative stress also contributed to the degeneration of DA neurons in DicerCKO mice.