RB1 and colonic neoplasm: On the other hand, in both human colon cancer cells and normal mouse colon epithelium, PXR/SXR protects against oxidant-induced apoptosis [153] and inhibits the proliferation and tumorigenicity of HT29 colon cancer cells by controlling the cell cycle at the G0/G1 cell phase by regulating p21WAF1/CIP1 and E2F/Rb pathways [154].