SPP1 and neoplasm: These include (i) the use of OPN as a prognostic biomarker for different tumors and its use as a predictor of therapeutic efficacy, (ii) the design of OPN-based targeted therapy specific for different tumor stages to improve treatment efficacy, and (iii) the use of OPN to monitor the occurrence of disease relapse during follow-up, since blood sampling is a fast, minimally invasive, and readily repeatable procedure.