SPP1 and cancer: The combination of OPN with other cancer-specific markers, synergizing different biomolecules (i.e., fibrinolytic system, calcium homeostatic proteins, squamous cell carcinoma antigen, NF-κB pathways, proteases, etc.), and functionally convergent elements (i.e., angiogenesis, motility/adhesion, extracellular matrix, bone, etc.)have been studied [39].