Nerve growth factor receptor contains an extracellular domain containing four 40-amino acid repeats with 6 cysteine residues at conserved positions followed by a serine/threonine-rich region, a single transmembrane domain, and a 155-amino acid cytoplasmic domain. The cysteine-rich region contains the nerve growth factor binding domain. NGFR p75 plays a central role in the regulation of cell number by apoptosis in the developing CNS. During early development, activation of NGFR p75 by NGF induces apoptotic cell death in some neuronal cells, probably through activation of the sphingomyelinase/ceramide pathway, the ICE like proteases and the JNK pathway. In rat Schwann cells, NGF binding to NGFR p75 activates NF kappaB, possibly to modulate Schwann cell migration during nerve regeneration. CD271 has recently been described as being expressed in mesenchymal stem cells (bone marrow stromal cells).