The neurotrophins utilize a complex receptor system consisting of Trk receptor tyrosine kinases and the structurally unrelated tumor necrosis factor receptor family member, p75 N T R , to confer diverse and sometimes opposing biological actions. The recent identification of selective ligands for p75 N T R , novel isoforms of this receptor, as well as new signaling partners, suggest that the numerous biological actions of the neurotrophins via p75 N T R may reflect selectivity of ligand-receptor interactions and intracellular adaptor protein recruitment.