An important puzzle in the field of PD research is why mice lacking PINK1 or parkin bear only subtle phenotypes related to dopaminergic neuronal degeneration or mitochondrial morphology change (Palacino et al., 2004; Perez and Palmiter, 2005; Perez et al., 2005; Kitada et al., 2007; Frank-Cannon et al., 2008; Gautier et al., 2008; Gispert et al., 2009; Kitada et al., 2009; Akundi et al., 2011). The gene discussed is PRKN; the disease is Parkinson disease.