CARD9 and colitis: For example, Trp of essential amino acids (EAAs) exerts beneficial regulatory function in mucosal growth or maintenance and alleviation of intestinal inflammation by 5-hydroxytryptophan (5-HT) signaling pathway [10], in the recovery of colitis by caspase recruitment domain family member 9 (Card9) [11, 12], and in the function of intestinal homeostasis and anti-inflammation by aryl hydrocarbon receptor (AHR) ligands in the intestine [13, 14].