PRKN and Parkinson disease: Interestingly, mice carrying PD-linked mutations, such as α-synuclein overexpressed under the Thy1 promoter (Lam et al., 2011), or parkin and DJ-1 double knock-out mice (Hennis et al., 2014) were found to be hyperactive and have increased striatal DA levels in their pre-symptomatic phase, possibly indicating compensatory mechanisms preceding nigro-striatal DA system demise.