SPP1 and cancer: It has been reported that a secretory isoform of OPN induces the EMT, while its intracellular/nuclear isoform induces the MET.35 It has also been reported that cancer cells express different splice variants.36 While OPN may facilitate cellular adhesion and contribute to the migration and invasion of tumor cells,37,38 it is also possible that an excessive amount of OPN inhibits cellular migration because of strong tumor cell attachment to the substrate surface.