SPP1 and neoplasm: Through the RGD cell adhesion sequence, OPN may interact with important tumor metastasis factors, including integrin, CD44, vascular endothelial growth factor/epidermal growth factor receptor (VEGF/EGFR), matrix metalloproteinases (MMPs), fibronectin (FN), survivin, transforming growth factor (TGF), tumor necrosis factor (TNF) and urokinase-type plasminogen activator (uPA) to promote cell chemotaxis, adhesion and migration (8–17).