Sonic hedgehog (Shh) is a member of the Hedgehog family of secreted signaling proteins that has a prominent role in patterning during early mammalian development (Ho and Scott). It is expressed during early embryogenesis in a variety of tissues (Finco et al.). During patterning of the central nervous system, Shh directly acts on cells to specify neural cell fates, regulates proliferation and survival of oligodendrocytes precursors and neural crest cells and stimulates proliferation, differentiation and axon growth (Ho and Scott; Martí). Shh also regulates T cell differentiation and activation (Crompton et al.). Recombinant human Shh (C24II; rhShh) is a fully biologically active molecule.