The Rb/E2F pathway is deregulated in virtually all human tumors. It is clear that, in addition to Rb itself, essential cofactors required for transcriptional repression and silencing of E2F target genes are mutated or lost in cancer. To identify novel cofactors required for Rb/E2F-mediated inhibition of cell proliferation, we performed a genome-wide short hairpin RNA screen. In addition to several known Rb cofactors, the screen identified components of the Mediator complex, a large multiprotein coactivator required for RNA polymerase II transcription. We show that the Mediator complex subunit MED13L is required for Rb/E2F control of cell growth, the complete repression of cell cycle target genes, and cell cycle inhibition.