LRRK2 and Parkinson disease: Knock-in (KI) models expressing a PD-associated LRRK2 mutation at endogenous levels did not show overt DA neuronal cell loss, but the mice at old age showed altered dopamine homeostasis, dysregulation of dopamine transport and accumulation into the synapse, and mild behavioral deficits, which are related to the prodromal phase of PD [53,54].