SPP1 and neoplasm: Furthermore, the association of osteopontin overexpression both with tumour progression and with metastasis has been documented in other in vivo animal models (Craig et al, 1990; Gardner et al, 1994; Feng et al, 1995; Behrend et al, 1995) and in human studies (Singh et al, 1997; Tuck et al, 1997; Rudland et al, 2002).