SPP1 and breast cancer: Osteopontin (OPN), a secreted multifunctional phosphorylated protein [60] present in milk from cows, buffalos, sheep, goats, and yaks [316], has also been identified to be highly expressed in the tumor tissue and plasma of BC patients, including TNBC tissues and cells, in association with a poor clinical prognosis and reduced survival, while OPN downregulation inhibits BC skeletal metastasis in vitro [317,318].