The first evidence of the presence of CSCs in solid cancer in vivo was found and identified as CD44+CD24-/lowLineage− cells in immunocompromised mice after transplanting human breast cancer cells in 2003 [27] even though it has been indicated in vitro in 2002 by the discovery of clonogenic (sphere-forming) cells isolated from human brain gliomas [28]. Here, CD44 is linked to breast carcinoma.