SPP1 and neoplasm: OPN is expressed by many cell types such as bone cells (e.g., osteoblasts, osteoclasts, and osteocytes), immune cells (e.g., T cells, B cells, natural killer cells, and macrophages), neural cells, epithelial cells, fibroblasts, smooth muscle cells, and endothelial cells [34–36] and in several tumor-derived cell lines.