SPARC, also referred to as osteonectin or basement membrane protein 40 (BM-40), is a ~32 kDa calcium-binding matricellular protein [PMID: 22249026, PMID: 25064829]. At one time SPARC was thought to be expressed specifically in mineralized tissues (thus the name osteonectin) but was subsequently demonstrated to have a much wider expression pattern in both mineralized and non-mineralized tissues [PMID: 7034958]. Typically, expression of SPARC is closely aligned with that of fibrillar collagens such as collagen type I. The protein component of the pre-mineralized bone matrix (osteoid) is composed mainly of collagen type I (94%) and a number of noncollagenous proteins.