CD44 and cancer: Full-length CD44 was thought to be an ideal stem cell marker,24,25 but recently, the CD44v6 splice variant has been reported to be a more specific marker.26,27 In addition, some cancer stem cell markers are derived from mouse tumor cells and have not been validated in human samples.20 Thus, it is unclear whether these markers can be used to identify all human cancer initiating and sustaining cells.