Osteopontin (SPP1) is a highly phosphorylated sialoprotein that is a prominent component of the mineralized extracellular matrices of bones and teeth. It binds multiple integrins, including alphaVbeta3, alphaVbeta1 and alphaVbeta5 (Liaw et al. 1995) alpha9beta1 (Smith et al. 1996, Yokosaki et al. 1999), alpha4beta1 (Bayliss et al. 1998) and the receptor CD44 (Katagiri et al. 1999).