DAZ is an RNA-binding protein encoded by a region on the Y chromosome implicated in infertility, and DAZ-like (Dazl) proteins are master regulators of germ line gene expression in all animals. In mice Dazl is only expressed in germ cells and is necessary for meiosis. A dual approach was taken to understand the RNA-binding specificity of the Dazl protein: (i) traditional SELEX and (ii) a novel tri-hybrid screen. Both approaches led to the same conclusion, namely that Dazl binds oligo(U) stretches interspersed by G or C residues. In a directed tri-hybrid assay the strongest interaction was with the consensus (GUn)n. This motif is found in the 5' UTR of CDC25C whose homologue is thought to be the target of Boule, the Dazl homologue in flies. CDC25C 5' UTR also interacted specifically with Dazl in vitro. The tri-hybrid screen retrieved UTRs of known genes that may be physiological substrates of Dazl.