Osteopontin, also known as early T-lymphocyte activation 1 protein (ETA1), is a member of the SIBLING (Small Integrin-Binding Ligand, N-linked Glycoprotein) proteins that are secreted by several cell types, such as bone cells (i.e., osteoblasts, osteoclasts and osteocytes) and inflammatory cells (e.g., macrophages); osteopontin is a substrate of MMPs (MMP-9 and, possibly, MMP-2), and tartrate-resistant acid phosphatase (TRACP), which is increased in Gaucher disease [30,31]. The gene discussed is SPP1; the disease is Gaucher disease.