Secreted phosphoprotein 1 encoded by SPP1 is involved in the attachment of osteoclasts to the mineralized bone matrix. Secreted phosphoprotein 1 is secreted and binds hydroxyapatite with high affinity. The osteoclast vitronectin receptor is found in the cell membrane and may be involved in the binding to secreted phosphoprotein 1. Secreted phosphoprotein 1 is also a cytokine that upregulates expression of interferon-gamma and interleukin-12. Several transcript variants encoding different isoforms have been found for SPP1.