CD4+ T cells can differentiate into effector (Th1, Th2, Th17, and follicular Th (Tfh) cells) or regulatory phenotypes (forkhead box protein (Foxp)3+ regulatory T cells (Tregs) and type 1 regulatory T cells (Tr1)); further, these cytokine-producing Th subsets including Th1 (interferon (IFN)-γ), Th2 (IL-4, IL-5, IL-13), Th9 (IL-9), Th17 (IL-17), Th22 (IL-22), Treg (IL-10), and Tr1 (IL-10) cells, which are all involved in the development of IBD. The gene discussed is IL9; the disease is inflammatory bowel disease.