SPP1 and cancer: Then, to evaluate the effect of kinase-mediated phosphorylation in cancer-secreted OPN on cancer cell migration, we performed a migration assay using MDA-MB435S CM with or without kinase treatment because the OPN in MDA-MB435S CM was less phosphorylated (Figure 4), allowing the assay to ignore the effect of endogenous phosphorylated OPN (Figure 6c,d).