CD44 and breast cancer: In 2003, Al-Haji et al discovered that as few as 100 CD44-positive human breast cancer cells can form new tumors.11 Accumulating evidence from various types of cancer has led to CD44 being widely recognized as a key molecular marker of CSCs.12,13 In addition to its role as an adhesion molecule in cell-matrix interactions, CD44 plays various roles that support the maintenance of stem cell properties.