In this scenario, both the reported interaction of LRRK2 with parkin [16] and the recent observations of several subjects simultaneously harboring both LRRK2 and PRKN mutations [17–19] have invited to consider a potential genetic interplay between these two most common genetic causes of parkinsonism in the mechanisms of penetrance, with a model of gene–gene interaction. The gene discussed is PRKN; the disease is Parkinsonism.