 It is hypothesised that maintenance of commensal microorganism growth in the GI tract is dysregulated, either as a result or cause of immune dysregulation.A number of studies have suggested a causal role for "Mycobacterium avium" subspecies "paratuberculosis" (MAP), which causes a similar disease, Johne's disease, in cattle.NOD2 is a gene involved in Crohn's genetic susceptibility.