CXCR4 and small cell lung carcinoma: The importance of CXCR4 in the process of cancer metastasis is evidenced by its overexpression in variety of tumors like small-cell lung carcinoma, prostate cancer, pancreatic cancer, breast cancer, gastric cancer, glioma, colon carcinoma, acute myeloid leukemia, ovarian cancer, melanoma, as compared to normal cells [19,24,65,66,67].