SPP1 and central nervous system cancer: OPN has been detected in a growing number of human tumour types, including lung, breast, prostate, gastric, oesophageal, ovarian and glioma, by immunohistochemistry on tumour tissue sections, quantification of OPN RNA from tumour tissue or in expression array studies from tumour tissues (reviews: Furger et al, 2001; Tuck and Chambers, 2001).