Stem cell migration is regulated by specific chemokines and their receptors [94], and one of these receptors, CXCR4, was found to be overexpressed in metastatic breast cancer [95], leading to the conclusion that tumorigenic stem cells are not only the origin of primary tumors, but might also be responsible for metastases as the result of tumor cell homing and growth at sites removed from the parent tumor [88,96]. This evidence concerns the gene CXCR4 and neoplasm.