The transcription factors encoded by the MYC genes control diverse tumorigenesis-relevant processes such as cell-cycle progression, growth factor dependence, and response to anti-mitogenic signals (for a review, see Refs. 1 and 2). Overexpression of one of the MYC genes as a result of chromosomal translocation, gene amplification, or loss of negative transcriptional control plays a prominent role in the etiology of many types of tumors (for a review, see Refs. 3 and 4). The MYCN gene is found amplified in several tumors of mostly neuroendocrine origin including about 25% of neuroblastomas, the most common solid tumor in childhood (for a review, see Refs. 5 and 6). Whereas many neuroblastomas regress spontaneously or can be cured with minimal therapy, MYCN-amplified tumors have a poor prognosis. The treatment of these patients has not improved significantly in the course of the last two decades. Recently, the comparison of the gene expression profiles of MYCN-expressing versus non-expressing neuroblastoma cells as well as MYCNamplified versus non-amplified primary tumors have begun to address the functional consequences of the massive overexpression of MYCN resulting from gene amplification (7,8).Several mouse models of Myc-induced tumorigenesis suggest that Myc is not only required for the initiation but also for the maintenance of the tumorigenic state supporting the value of the MYC genes as targets of tumor therapy (9 -11). Roughly half of the drugs that are in clinical use currently act as inhibitors of enzymes. In transcriptional regulation, enzymes are involved mainly as components of signal transduction cascades that relay information to the promoter and as transcriptional co-regulators that modulate local chromatin structure either by covalent modification of histones or by an ATPase-dependent remodeling of nucleosomes (for a review, see Ref. 12). The efficacy of histone deacetylase inhibitors against leukemia has proven the principle of treating cancer by the selective pharmacological modulation of the transcription machinery (13). Application of this concept to the MYCN gene requires a detailed knowledge of the molecular basis of the transcriptional activation of MYCN in neuroblastomas.Toward this goal, we have recently identified the activating members of the E2F family of transcription factors (E2F-1, E2F-2, and E2F-3) as regulators of MYCN expression in neuroblastomas (14). E2F proteins are important regulators of cell-cycle progression, and their activity is negatively controlled by the p16/Rb pathway, which is inactivated in the majority of human cancers (for a review, see Refs. 15 and 16). Although genetic defects of the p16/Rb pathway are rare in neuroblastomas, there is some evidence for a loss of normal control of E2F activity in neuroblastomas by epigenetic means (17).In addition to E2F binding sites, the MYCN promoter contains several putative binding sites for Sp1 and related zinc finger transcription factors (18). One of these, a non-consensus binding site, the CT-box, was previously impli...