Nature 412, 355–358 (2001).

In this Letter it was reported that an amino-terminal CDK-inhibitory domain of the Cdc6 replication protein was required for efficient mitotic exit, and that removal of this domain lethally blocked mitotic exit in cells lacking the Sic1 CDK inhibitor, demonstrating essential CDK-inhibitory control of CDK activity. However, Archambault et al.1 have shown that viable mitotic-exit-competent strains lacking both CDK-inhibitors can be constructed, a finding that has also been confirmed by the authors. Archambault et al.1 suggest that CDK-inhibitors are not required for mitotic exit in the wild-type cell cycle. However, A.C. et al. (manuscript in preparation) have evidence that strains lacking both CDK inhibitors may play a non-essential role in the regulation of mitotic exit. This point is still under investigation.