ErbB4, a key cellular receptor for neuregulin growth factors, is induced in the colon in IBD.17 Genome-wide association studies link single-nucleotide polymorphisms in ErbB4 to both Crohn's disease and ulcerative colitis.18 Interestingly, the specific ErbB4 ligand neuregulin-4 (NRG4) is suppressed in disease,19 whereas exogenous NRG4 is protective in rodent models of intestinal inflammation.19, 20 Taken together, these results suggest a protective role for the ErbB4 signaling axis, which is disrupted in IBD. Here, NRG4 is linked to Crohn disease.