A mouse line was generated that expressed a gene trap reporter construct, betageo, in a dynamic pattern during embryonic development. Differential expression was seen within the developing eyes, limbs, heart, neural tube, and skeleton. Two transcripts were cloned that contained endogenous sequences fused to the gene trap vector sequence. Analysis of the endogenous sequences revealed that the reporter integrated within a gene belonging to a small group of eukaryotic superfamily I helicases. Unexpectedly, the majority of transcripts produced from the trapped locus were not affected by the insertion of the reporter. Although the function of the trapped helicase gene is unknown, its complex transcription patterns and widespread spatial-temporal distribution suggest that the gene product plays a role in RNA metabolism in multiple tissues and organs within the developing embryo.