LRRK2 and Parkinson disease: LRRK2 mutations have an incomplete penetrance in PD patients and most rodent transgenic models developed to date (http://www.neurodegenerationresearch.eu/models-for-parkinsons-disease/in~vivo-mammalian-models/lrrk2/) show no or little evidence of any neuronal cell loss, strongly indicating that other factors, environmental or genetic, must intervene to trigger neuronal cell loss.