SPP1 and neoplasm: OPN also acts as a ligand for CD44 that results in attracting mesenchymal stem cells (MSCs) to the tumor microenvironment, promoting EMT and tumor budding (TB), inducing macrophage migration and activation, stimulating intestinal growth, differentiation and maturation, cell growth, proliferation, migration and cell-cycle activity and promoting oxidative stress [15,17,23,33,34,35,36].