The N-myconcogene directs organogenesis, and gene amplification is associated with aggressive forms of neuroblastoma, a common malignant tumor in children. N-mycis expressed in fetal epithelium, and expression decreases markedly postnatally. To localize sequences responsible for directing expression, we have analyzed the human N-mycpromoter. We noted previously that N-mycpromoter regions 5′ to exon 1 directed reporter gene expression in all cell lines, including those without detectable N-myctranscripts. However, when promoter constructs included 3′ exon 1 and the 5′ portion of intron 1, reporter activity was detected only when there was expression of the endogenous gene. To determine the role of this "tissue-specific region" in directing expression during development, we generated transgenic mice carrying N-mycpromoterlacZminigenes that contained 5′ N-mycpromoter elements alone or the promoter linked to the 3′ axon 1/5′ intron 1 tissue-specific region. Animals lacking the tissue-specific exon 1/intron 1 region showed β-galactosidase expression in the CNS, but expression was not observed in other organs in which endogenously derived N-myctranscripts were seen. Within the CNS, transgene expression was seen mainly in the olfactory system and was not observed in other areas in which expression of the murine gene has been noted. In contrast, no transgene expression was observed in any of the animals carrying the tissue-specific exon 1/intron 1 region. Thus, sequences that direct expression within the olfactory system were contained within our 5′ promoter transgene, whereas sequences that guide the ubiquitous expression of N-mycduring organogenesis lie outside the regions studied here. Finally, the exon 1/intron 1 region seems to act in a dominant fashion to repress expression in the CNS from the immediate 5′ N-mycpromoter.