CD44 and esophageal squamous cell carcinoma: In this study, we also detected CTCs with combined expression of EpCAM and CD44, which is one of the other proposed CSC markers in ESCC [19]; however, almost all the EpCAM+ cells were CD44+, indicating that CD44 is almost equivalent to EpCAM as a CTC marker and therefore it is not useful to differentiate subpopulations of CTCs in ESCC.