SPP1 and breast cancer: Kang et al. reported that elevated expression of pre-metastatic genes - C-X-C chemokine receptor 4 (CXCR4), matrix metalloproteinase (MMP)-1, connective tissue growth factor (CTGF), fibroblast growth factor (FGF5), interleukin (IL)-11, and osteopontin (OPN) – are collectively responsible for the manifestation of osteolytic breast cancer metastasis (Kang et al., 2003).