Several new members of the nerve growth factor family of neurotrophins, which comprises nerve growth factor itself, brain-derived neurotrophic factor and neurotrophins-3, -4 (also known as neurotrophin-5) and -6, have been isolated in recent years. Their signaling receptors have been identified as the Trk family of tyrosine protein kinases, thus facilitating the dissection of the signaling pathways responsible for mediating their trophic properties. More recently, the advent of gene targeting has made it possible to generate strains of mice lacking neurotrophins and their receptors. Analysis of the phenotypes of these mutant animals has provided detailed information on the role that neurotrophins and their receptors play in the ontogeny of the mammalian nervous system.