Over-expression of the transcription factor c-Myc immortalizes primary cells and transforms in co-operation with activated ras. Therefore, c-myc is considered a proto-oncogene. Since its discovery c-Myc has been shown to render cells growth factor independent, accelerates passage through G1 of the cell cycle, inhibits differentiation and elicits apoptosis. Whereas the effects on immortalization, proliferation and inhibition of differentiation are in conceivable accordance with gain of function, as it is defined for a proto-oncogene, its pro-apoptotic activity disables a straight forward explanation of the physiological role of c-Myc and suggests a highly complex contribution during development. The recent accomplishments in c-Myc research shed some light on the difficile regulatory network which keeps check on c-Myc activity such as by binding to proteins some of which are transcription factors for non-c-Myc targets. Moreover, it was shown that genes are targeted by c-Myc depending on the sequence of flanking regions adjacent to the E-box or in dependence on the availability of binding partners which is most probably specific to the cellular context. Cdc25A and ornithine decarboxylase, both described to be c-Myc targets, have been brought forward as downstream effectors in the induction of proliferation under serum rich conditions, or in the induction of apoptosis when serum factors are limited. These genes seem to be regulated by c-Myc in a cell type-specific manner. H-ferritin, IRP2 and telomerase are the most recently discovered direct targets of c-Myc. The regulation of H-ferritin and IRP2 might explain the potential of c-Myc to promote proliferation and the regulation of telomerase could be responsible for the immortalizing properties of c-Myc. In the future, H-ferritin and telomerase have to be analyzed whether or not these genes are also Myc targets in other cell systems. Although the intense research efforts regarding the function of c-Myc last already two decades the role of this gene is still enigmatic.
