The oncogenic activation by amplification of the MYCN gene is frequently observed in human neuroblastomas and occasionally in other tumours with neuronal qualities. As a consequence of amplification, elevated levels of the mycN protein are expressed. mycN contains a C-terminal basic region (BR) that can bind to DNA, and a helix-loop-helix (HLH)-leucine zipper (Zip) domain, which is responsible for the physical interaction with another HLH-Zip protein, max. This principle structure is conserved among all members of the MYC gene family. The resulting dimers can bind to the DNA sequence CACGTG. The mycN protein, but not max, contains, near the N-terminus, a region conferring the ability to activate the transcription of genes. mycN/max heterodimers probably activate and max/max homodimers repress transcription of, as yet, unidentified target genes. In neuroblastoma cells, where mycN is deregulated, the balanced interaction of BR-HLH-Zip proteins is probably perturbed, and, therefore, genes controlled by mycN might be abnormally expressed and thereby alter normal cell growth with the consequence of tumorigenesis.