POSTN and osteosarcoma: The functional role of periostin in cancer is also under debate as both tumor-promoting [5-7,10,20,21] and tumor-suppressing activities [18,19] have been reported: on one hand, periostin was reported to increase invasiveness of tumor cell lines in vitro [7,12,20] but, on the other hand, periostin expression reduced invasiveness of bladder cancer cells [18] and decreased anchorage-independent growth of T24 bladder cancer cells and SaOS-2 osteosarcoma cell line [19].