RAC1 and intestinal neoplasm: By putting these two complementary studies together, we propose that Rac1-dependent superoxide is required for expansion and transformation of intestinal stem cells in the crypt; whereas PrxII-regulatable H2O2 intimidates the tumorigenic epithelial cells by targeting TNKS in growing intestinal tumors and is not necessary for classical Wnt/β-catenin signaling.