The finding that the mAb 446 inhibition ELISA and the commercial OPN kit could not detect elevated OPN levels in the cancer patients suggested the possibility that the OPN present could be fragmented i.e. the target sites for the antibodies used in these assays could be missing, whereas, by virtue of the design of the mAb 659-based assay, the thrombin-sensitive site must be present. This evidence concerns the gene SPP1 and cancer.