NOD2 and Crohn disease: An important role for host–microbial interactions is further highlighted by Crohn's disease (CD)-associated loss-of-function polymorphisms in NOD2 (refs 1, 2; encoding NOD2, a PRR that recognizes bacterial peptidoglycan) as well as in additional pathways regulating microbial clearance mechanisms, such as autophagy3 and NADPH complex-mediated generation of reactive oxygen species (ROS)4, 5.