SPP1 and cancer: Osteopontin (OPN) is a secretory phosphoprotein expressed in various tissues, including the brain, and it has been reported that OPN has important roles in various physiological processes, such as anchoring osteoclasts to the mineral bone matrix during bone resorption1 and attaching calcium oxalate crystals to renal tubules in stone formation.2 Elevation of OPN expression has been observed in various pathological conditions, including atherosclerosis,3 multiple sclerosis,4 rheumatoid arthritis5 and most cancers,6 which suggests that OPN has an important role in these conditions.