We have identified daf-5 as W01G7.1, a predicted gene related to the Sno family of oncogenes. Loss-of-function mutations in daf-5 result in a dauer-defective (Daf-d) phenotype. They have been shown to suppress dauer-constitutive (Daf-c) mutations in the TGF-beta signaling branch of the dauer pathway, and to enhance the life span extension of daf-2 (insulin receptor) mutants. Genetic epistasis tests have placed daf-5 downstream of the daf-1 type I and daf-4 type II TGF-beta receptors and at the same step as the Daf-d gene, daf-3 , which encodes a SMAD transcription factor (Patterson et al., Genes & Development 11: 2679-2690, 1997). This extends the parallel between dauer signaling and mammalian TGF-beta signalling, in which the SnoN oncoprotein has been shown to interact directly with SMAD transcription factors (Stroschein et al, Science 286: 771-774, 1999). daf-5 was physically mapped using DNA dimorphisms between the strains N2 and RC301 to refine its position prior to DNA transformation experiments. The cosmid W01G7 was found to rescue daf-5 , using the elimination of suppression of a daf-7 Daf-c mutation as the assay for daf-5(+) activity. W01G7.1 RNAi treatment of daf-7 resulted in strong suppression of the Daf-c phenotype, indicating that this candidate gene corresponds to daf-5 . We then sequenced several alleles of daf-5 , finding alterations that result in premature stops or amino acid substitutions. Southern analysis of a putative Tc1-insertion allele ( m512 ) using a cDNA probe demonstrated that the entire locus is deleted, and that m512 is a null. It exhibits the same phenotype as the other Daf-d alleles. The mutation resulted from excision of a resident Tc1 element adjacent to daf-5 in the parent mutator strain. PCR analysis of mutant genomic DNA showed that the genes flanking daf-5 are intact. The cDNA was sequenced in its entirety, confirming all predicted splice sites, and RT-PCR experiments indicate that daf-5 is trans-spliced to SL1. GFP reporter studies showed that the daf-5 promoter is active in neurons in the head, consistent with the expression patterns of the daf-1 and daf-4 receptors and the daf-3 SMAD. daf-3 and daf-5 mutants have no longevity phenotype on their own, but they enhance the life span of daf-2 mutants, showing that neural TGF-beta signaling plays a minor role in determining adult life span that is only revealed in a daf-2 background.