NRG4 and inflammatory bowel disease: We have previously shown that in human IBD and chronic mouse colitis, expression of the ErbB4-specific ligand NRG4 is lost, potentially leading to a dysregulated ErbB4 signaling axis.19 NRG4 is most prominently expressed in the mesenchyme of the colon,19 though Feng and Teitelbaum have also detected expression in epithelium27 and we have detected regulated expression in enteroids and immune cells (unpublished results).