The Myb proto-oncogene family is recurrently mutated in human leukemia, brain cancer, breast cancer, and cancer of the salivary gland. Whereas vertebrate animals have three different Myb genes, Drosophila has a single non-redundant gene. The Drosophila Myb protein regulates gene expression in concert with other proteins in the RBF E2F2 and Myb-interacting proteins (dREAM) complex. Apart from Myb, homologs of all proteins contained within the dREAM complex were identified by mutations in the group B synthetic multivulval (synMuvB) genes in C. elegans. In humans, the MuvB core associates either with Rb-E2F-Dp proteins or with Myb proteins. In the absence of Myb the MuvB core suppresses the transcription of other genes during G0/G1 of the cell cycle, while B-Myb is bound to the MuvB core during S/G2 of the cell cycle and activates transcription. The Myb-interacting protein of 120 kDa (Mip120, homologous to LIN54 in humans and C. elegans), is part of the MuvB core. While Myb and other components of the dREAM complex are misregulated or mutated in many human cancers, a role for Mip120 remains unclear. In the course of studying the epigenetic regulation of Drosophila eye development by the dREAM complex, we have discovered a new role for Mip120 in tumorigenesis. Dual expression of two members of the dREAM complex, Myb and Mip120, caused solid tumors in 3rd instar larvae, while expression of Myb or Mip120 alone did not have this effect. This result suggests that protein stoichiometry and abundance of interactors of the dREAM complex play a vital role in regulation of tumor development. Our work also highlights the role of Mip120 as a critical member of the Myb-MuvB/dREAM complex in oncogenesis.