Studying breast cancer-derived cell lines, Stivarou et al. found that extracellular HSP90 was overexpressed in mammosphere-forming cell cultures, while the mammospheres were enriched with CSC-like CD44(+)/CD24(−/low) cells; the authors suggested that extracellular HSP90 represents a phenotypic marker of breast CSCs [106]. Here, CD44 is linked to breast cancer.