To test the effects of Fas signaling on TH9 cell apoptosis and proliferation in vivo, we transferred carboxyfluorescein succinimidyl ester (CFSE)-labeled WT-TH9 or Faslpr-TH9 into IBD mice and evaluated apoptosis and proliferation in the CFSE-positive cells in the mesenteric lymph nodes. Here, FAS is linked to inflammatory bowel disease.