SMAD4 and Familial prostate cancer: Loss of miR-15 and miR-16 in prostate cancer cells potentiates TGF-β signaling by upregulating USP9X (a gene encoding an enzyme deubiquitinating SMAD4), as well as activin RIIA, an activin receptor, contributing to the survival of cancer cells in bone marrow and the formation of bone metastasis.