SPP1 and cancer: These OPN fragments, which by inference must bear the intact thrombin-sensitive site, are probably truncated somewhere at the N-terminal end of the protein based on the observation that two immunoassays used in parallel with the mAb 659 inhibition ELISA had failed to detect similar OPN increases in the cancer patients: (1) An inhibition ELISA which uses mAb 446 as the detecting reagent known to bind to the N-terminal half of OPN (actual location unknown).