Differentiation, survival, and function of the vertebrate neurons are controlled by multiple, target-derived neurotrophic factors. The best characterized mammalian neurotrophic factors are four structurally related 13 to 14 kDa basic proteins, collectively known as neurotrophins. Here we describe the identification of a gene cluster localized on human chromosome 19 that contains neurotrophin-5 (NT-5) and that may encode three additional acidic members of this protein family. The three novel partial open reading frames (ORFs), designated neurotrophin-6-alpha (NT6-alpha), NT6-beta and NT6-gamma, are 95% identical to each other and 75% identical to NT5. The putative mature N-terminal portion of NT6 ORFs does not contain a typical dibasic cleavage site and lacks two out of six cysteines that are conserved among the neutrophins. The unique structures of NT6-alpha, -beta, and -gamma suggest that if the NT6 open reading frames indeed code for functional proteins, these proteins may display novel functions and may act through a distinct class of receptors. In the human, both NTF5 and NTF6 gene loci were mapped to chromosome 19 by Southern analysis of somatic cell hybrid panels. In mouse, the NT5 gene (Ntf-5) was assigned to chromosome 7 and no sequences representing NT6 homologs were identified.