ALDH1A1 and neoplasm: It has been reported that CD133, CD44, and ALDH1A1 were present at low numbers in primary tumors, however, this was found to increase in tumor specimens taken immediately after chemotherapy treatment and then reduced to initial numbers in recurrent tumors, suggesting that these so-called “CSC markers” identify “chemoresistant cells.” Such observations in animal models and clinical specimens suggest that chemotherapy treatment may induce a “chemoresistant niche,” which protects residual chemoresistant cells from cell death by promoting a microenvironment appropriate for the survival of CSCs.