Members of the MYC gene family, including MYC, NMYC, and LYMC, have been found amplified and expressed at high level in various human cancers. We have analysed the expression of two members of the MYC gene family, NMYC and MYC, in human and murine neuroblastoma cells. Whenever NMYC and MYC are co-expressed, MYC expression predominates. Cells carrying high expression of NMYC as result of amplification lack MYC expression. The same is ture for neuroblastoma cells in which expression of a single-copy NMYC is upregulated or into which a vector forcing high expression of an exogenous NMYC had been introduced by transfection. Our studies indicate a regulatory interaction between MYC and NMYC in neuroblastoma cells.