CD44 and breast cancer: BCSCs are classically defined by CD44 (Cluster of Differentiation antigen-44) positive and low or absent levels of CD24 (Cluster of Differentiation antigen-24) expression (CD44+/CD24−/low) on their surface [47] and recent clinical evidence has established that tumorigenic BC cells with high expression of CD44 and low expression of CD24 are resistant to chemotherapy [48].