Tumor cells induce the differentiation of osteoclasts from mononuclear precursors by producing osteoclast-stimulating factors or by engaging other stromal cells, such as osteoblasts (9–17); in turn, osteoclasts digest bone matrix, leading to the release of embedded growth factors, such as TGF-β, IGF, and calcium, to facilitate tumor cell seeding and proliferation (18–24). The gene discussed is TGFB1; the disease is neoplasm.