CD44 and breast cancer: Although it has been reported that CD44+ and PROCR+ cells were similar to each other and were enriched for genes involved in cell motility, chemotaxis, hemostasis, and angiogenesis as well as stem cell-specific genes [22], our data suggested that PROCR+/ESA+ allowed further enrichment of highly tumorigenic cancer stem cells from the CD44+/CD24−/low breast cancer cells.