Cell death within the developing vertebrate nervous system is regulated in part by interactions between neurons and their innervation targets that are mediated by neurotrophic factors. These factors also appear to have a role in the maintenance of the adult nervous system. Two neurotrophic factors, nerve growth factor and brain-derived neurotrophic factor, share substantial amino acid sequence identity. We have used a screen that combines polymerase chain reaction amplification of genomic DNA and low-stringency hybridization with degenerate oligonucleotides to isolate human BDNF and a human gene, neurotrophin-3, that is closely related to both nerve growth factor and brain-derived neurotrophic factor. mRNA products of the brain-derived neurotrophic factor and neurotrophin-3 genes were detected in the adult human brain, suggesting that these proteins are involved in the maintenance of the adult nervous system. Neurotrophin-3 is also expected to function in embryonic neural development.