The MDM2 protein, through its interaction with p53, plays an important role in the regulation of the G(1) checkpoint of the cell cycle. In addition to binding to and inhibiting the transcriptional activation function of the p53 protein, MDM2 binds, inter alia, to RB and the E2F-1.DP-1 complex and in so doing may promote progression of cells into S phase. Mice transgenic for Mdm2 possess cells that have cell cycle regulation defects and develop an altered tumor profile independent of their p53 status. MDM2 also blocks the growth inhibitory effects of transforming growth factor-beta1 in a p53-independent manner. We show here that a novel growth regulatory molecule is also the target of MDM2-mediated inhibition. Using a yeast two-hybrid screen, we have identified a gene that encodes a novel cellular protein (MTBP) that binds to MDM2. MTBP can induce G(1) arrest, which in turn can be blocked by MDM2. Our results suggest the existence of another growth control pathway that may be regulated, at least in part, by MDM2.