The seminal study by Al-Hajj et al. [4] showed that CD44+/CD24−/low/lin− (lack of expression of CD2, CD3, CD10, CD16, CD18, CD31, CD64 and CD140b) breast cancer cells obtained mainly from pleural effusions could form tumors when as few as 200 were injected into the mammary fat pad of nonobese diabetic (NOD)/severe combined immunodeficient (SCID) mice. This evidence concerns the gene CD44 and breast cancer.