In the eukaryotic cell cycle, nuclear DNA replication (S phase) and mitosis (M phase) are linked such that replication must be complete before mitosis can begin. In order for this coupling to work, there must be some system for detecting unreplicated DNA and transducing an inhibitory signal to prevent the activation of mitotic factors. The DNA-bound protein RCC1 is involved in this regulatory process since mitosis initiates before DNA synthesis is finished in the absence of RCC1. This has led to the proposal that RCC1 is a signalling molecule, detecting unreplicated DNA and producing the inhibitory signal. However, mutants in RCC1 show defects beyond their inability to regulate the cell cycle, suggesting other roles for the RCC1 protein in the nucleus and thus hitherto unexplored relationships between cell cycle control and other cellular processes.