Osteopontin (SPP1) is a highly phosphorylated sialoprotein that is a prominent component of the mineralized extracellular matrices of bones and teeth. It provides an adhesive matrix for endothelial and smooth muscle cells during remodeling of the vascular wall following injury. SPP1 binds multiple integrins including alphaVbeta3, alphaVbeta1 and alphaVbeta5 (Liaw et al. 1995), alpha5beta1 (Barry et al. 2000), alpha9beta1 (Smith et al. 1996, Yokosaki et al. 1999) alpha4beta1 (Bayless et al. 1998), alpha8beta1 (Denda et al. 1998) and the receptor CD44 (Katagiri et al. 1999). Integrin alpha4beta1 is expressed on leukocytes, differentiated vascular smooth muscle cells and tumor cells. It has been shown to mediate leukocyte attachment to OPN.