SPP1 and cancer: The observation that OPN is involved in tumorigenesis, progression and metastatic dissemination of different types of human tumours, such as breast (Tuck et al, 1997; Tuck and Chambers, 2001), prostate (Thalmann et al, 1999), colon (Agrawal et al, 2002), ovarian (Kim et al, 2002), gastric (Ue et al, 1998) and lung (Chambers et al, 1996) cancer, has opened an interesting field of research, both for its possible prognostic significance in terms of disease-free and overall survival, and for its eventual role as a specific target in designing new anticancer therapies.