SPP1 and neoplasm: OPN is a secreted glycosylated phosphoprotein classified as a member of the ‘small integrin-binding ligand N-linked glycoproteins’ (SIBLINGs) and has been associated with tumorigenesis and metastasis of several tumor types (Yoon et al., 2002; Rangaswami et al., 2006; Das et al., 2009; Pietras et al., 2014; Ahmed et al., 2016; Zhao et al., 2018).