The expression of preprotachykinin-1 (PPT-1) is increased in breast cancer cell lines and in biopsy samples of malignant breast compared with normal mammary cells and together with its receptors (i.e., neurokinin-1 (NK-1)) and neurokinin-2 (NK-2) is associated with homing, integration, dysfunction in BM microenvironment, and eventual metastasis [33]. This evidence concerns the gene TAC1 and breast cancer.