SPP1 and breast cancer: Considered to be a canonical secretory protein, the multifunction of osteopontin (OPN) has been extensively studied in the past.6 High cytoplasmic OPN staining was observed in multiple tumors and the OPN level is closely correlated with the pathological stage.7 Blocking OPN in breast cancer cells was reported to decrease the expression of SNAIL, SLUG, and TWIST, which suggests that secretory OPN (sOPN) is critical in EMT and tumor metastasis.8