NOD2 and inflammatory bowel disease: For example, NOD2, the first gene to be associated with IBD, encodes an intracellular pattern recognition receptor that mediates the host immune response to the bacterial peptidoglycan product muramyl dipeptide (MDP) [26]; other IBD-associated genes with roles in anti-bacterial immunity include CARD9, IL23R, and LRRK2 [21–23].