LRRK2 and Parkinson disease: In conclusion, G2019S KI mice might represent a presymptomatic model of PD, a valuable tool to verify a “multi-hit” hypothesis of PD [78], where genetic variables (G2019S LRRK2), an established risk factor (aging), and internal (e.g. DA, α-syn) or environmental (e.g. MPTP) factors interact to shape the emergence of the parkinsonian phenotype.