MAX interactor 1 (Mxi1) proteins are c-Myc antagonists that primarily exert their biological functions by inhibiting Myc-dependent gene transcription. Mxi1 belongs to the Myc/Max/Mad network of proteins that have been implicated in the control of multiple aspects of cellular behavior. The Mxi1 protein functions in a regulatory network with members of the c-Myc family, in which c-Myc activates transcription and stimulates cell proliferation, and Mxi1 negatively regulates these actions. Inactivation of the MXI1 gene could, therefore, inhibit differentiation and enhance proliferation in the presence of normal levels of c-Myc, and thus MXI1 is a potential tumor suppressor gene. Max interactor-1 (MXI1), an antagonist of c-Myc that is involved in brain tumor progression, has been reported to be deregulated in a variety of tumors including glioma. Mxi1 is a transcriptional repressor and inhibits the proliferation of NB cells, suggesting that Mxi1 functions as an N-Myc antagonist.