In older studies, pan-Osteopontin (total Osteopontin, typically covering all variant forms) was measured.6,7 However, the gene product is subject to alternative splicing selectively in cancer, which deletes exon 4 (to generate Osteopontin-c) or exon 5 (to generate Osteopontin-b) from the unspliced form (called Osteopontin-a). This evidence concerns the gene SPP1 and cancer.