The first time the CSCs concept was applied to solid tumors was in 2003 by Al-Hajj and colleagues [78], when they identified that only a small subset of breast cancer cells expressing markers CD44+/CD24− possessed a marked proliferative capacity, differentiation, self-renewal, and in vivo tumor-forming ability, while the remaining bulk of cells from this tumor had none, even when injected at many-fold higher cell doses [79]. Here, CD44 is linked to breast cancer.