SPP1 (Osteopontin, BSP-1, ETA-1) is a protein found in the extracellular matrix of a number of immune cells. It is cleaved into an epitope (OPN-R) recognized by integrin receptors expressed on mast cells, neutrophils, T cells, monocytes and macrophages. It is involved in a number of biological processes including wound healing, cell fusion and migration, inflammation, immune response, bone turnover and remodeling, and tumorigenesis. OPN is also upregulated in a number of cancers and diseases. These include inflammatory bowel disease, rheumatoid arthritis, and lung, breast, colorectal, stomach, ovarian, and papillary thyroid carcinoma as well as melanoma. In immunohistochemistry of normal tissue, SPP1 has strong cytoplasmic positivity in the gallbladder and in renal tubules, and will also have variable staining in tissues throughout the body. References: Matrix Biol. 2015 Jan;41:19-25, PMID: 25446551; J Cell Physiol. 2000 Jul. 184 (1): 118–130, PMID: 10825241; Cytokine Growth Factor Rev. 2008. 19 (5–6): 333–45, PMID: 18952487.