In Al-Hajj’s study [7], as few as 1000 cells with CD44+/CD24-/low phenotype can give rise to tumors in all cases, while up to 2 × 104 CD44+/24+ cells fail to form tumors, which suggests that cancer cells with CD44+/CD24-/low phenotype have the tumorigenic capability upon serial xenotransplantation in vivo. Here, CD44 is linked to cancer.