SPP1 and breast cancer: These include the metastasis suppressor gene Raf Kinase Inhibitory protein (RKIP), targets of the downstream let-7 microRNA family including the pro-metastatic let-7 targets BACH1 and HMGA2, and finally their downstream targets MMP1, CXCR4 and OPN. We experimentally demonstrated that the microRNA let-7 suppresses breast cancer metastasis, and BACH1, a leucine zipper transcription factor, promotes breast cancer metastasis.