CD44 and colorectal cancer: Several CSC populations have been identified in colorectal cancer (CRC): (1) CD133+ CSCs, which have been isolated from primary colorectal tumors 8, 9; (2) ESA+/CD44+ CSCs, which are increased in xenogeneic colon tumors postchemotherapy 10; (3) CD26+ cells, which are enriched from CD133+/CD44+ populations and are capable of acting as metastasis-initiating CSCs 11; and (4) Lgr5+ CSCs, which are essential for the formation and maintenance of liver metastasis 12.