N-myc, a cellular gene related to the c-myc proto-oncogene^1–4, was originally identified on the basis of its very frequent amplification and overexpression in a restricted set of tumours, most notably human neuroblastomas^1,2. That N-myc may have a causal role in the genesis of these tumours is suggested by the observation that in the rat embryo fibroblast co-transformation assay^5,6 it has a transforming potential similar to that of c-myc^7,8. The apparent structural and functional homology of N-myc and c-myc suggests that they may be members of the same proto-oncogene family. However, despite these apparent similarities, expression of the two genes appears to be dramatically different with respect to tumour specificity, as well as tissue and developmental stage specificity^9,10. To further elucidate the common and unique aspects of N-myc and c-myc gene structure and function in normal and transformed cells, we have determined the organization of human N-myc and the nucleotide sequence of its messenger product, and we report here that N-myc and c-myc have a similar intron/exon structure and that their protein products share regions of significant homology.