SPP1 and neoplasm: OPN has been found to be present in three forms in tissues and fluids: i) an intracellular protein in complex with hyaluronan-CD44-ERM (ezrin/radixin/moesin) that is involved in migration of tumor and stromal cells [3]; ii) an extracellular protein that is abundant at mineralized tissues [4]; iii) a secreted protein that is found in fluids isolated from metastatic tumors [5] and also found in organs such as placenta [6,7], breast [8], and testes [9].