ABSTRACTThe cell cycle depends on a sequence of steps that are triggered and terminated via the synthesis and degradation of phase-specific transcripts and proteins. Although much is known about how stage-specific transcription is activated, less is understood about how inappropriate gene expression is suppressed. Here, we demonstrate that Groucho, theDrosophilaorthologue of TLE1 and other related human transcriptional corepressors, regulates normal cell cycle progressionin vivo. We show that, although Groucho is expressed throughout the cell cycle, its activity is selectively inactivated by phosphorylation, except in S phase when it negatively regulates E2F1. Constitutive Groucho activity, as well as its depletion and the consequent derepression ofe2f1, cause cell cycle phenotypes. Our results suggest that Cdk1 contributes to phase-specific phosphorylation of Grouchoin vivo. We propose that Groucho and its orthologues play a role in the metazoan cell cycle that may explain the links between TLE corepressors and several types of human cancer.