CD44 and colorectal cancer: Recent studies reveal that inhibition of TGF-β signaling in PCa cells impedes the PCa progression and corresponding bone metastases [11–13], as well as the discovery that external stimulation with TGF-β converted CD44- non-CSCs into the undifferentiated CD44+ CSCs in human colorectal cancer, leading to the significant increment of CSCs in xenograft models [14].