LRRK2 and Crohn disease: Three lines of research have led to a greatly improved understanding of PD: (1) pathologic staging of the pathology of PD by Braak [142,143], (2) the segregation of initiating PD stimuli to “body first” or “brain first” [144,145], and (3) the identification of polymorphisms of the LRRK2 and PARK genes associated with PD that are also associated with susceptibility to mycobacterial infection and Crohn’s disease [146,147].