CXCR4 and prostate carcinoma: Accumulating evidence indicates that prostate cancer and metastasis-initiating cells expressing stem-like markers including CD133+, CD44high, ALDhigh, ABCG2+ and CXCR4+high and endowed with a high self-renewal ability are critical for prostate cancer progression, metastasis and resistance to current clinical therapies [32, 37].