The evolutionarily conserved protein COP1 has been shown to operate as an E3 ubiquitin ligase complex, and a number of putative substrates have been identified, including the c-JUN oncoprotein and p53 tumor suppressor protein. New work by Migliorini and colleagues described in the current issue of JCI demonstrates that COP1 acts as a tumor suppressor in vivo and does so, at least in part, by promoting the destruction of c-JUN. These findings challenge the view that COP1 regulates p53 stability and call into question the wisdom of developing COP1 inhibitors as potential anticancer agents.