Among these interactions we mention: (i) integrin receptors with the RGD domain (Christensen et al., 2010); (ii) CD44 domain and collagen, with highly negative polysaccharides involved (Ponta et al., 2003); (iii) post-translational modifications activating different biochemical pathways (Weber et al., 2002), including biomineralization (Li and Wang, 2012); (iv) interactions with cations (Ca, Mg) modulating OPN activity (Hu et al., 1995); (v) OPN antibodies inhibiting the rheumatoid arthritis propagation (Du et al., 2008) (PDB 3CXD). The gene discussed is SPP1; the disease is rheumatoid arthritis.