CORRESPONDENCE

This is an open-access article distributed under the terms of the Creative Commons Attribution Licence, which permits distribution and reproduction in any medium, provided the original author and source are credited. Creation of derivativeworks is permitted but the resultingworkmay be distributed only under the same or similar licence to this one. This licence does not permit commercial exploitation without specific permission. The eukaryotic cell cycle comprises an ordered series of events that are controlled by the oscillating activity of cyclin-dependent kinases (Cdks). The unidirectionality of cell cycle transitions is fundamental for successful completion of this cycle. Increasing activity of Cdk1 in complex with its regulatory cyclin, CycB, drives entry into mitosis, whereas subsequent CycB proteolysis promotes exit from mitosis and return of the cell cycle to interphase (Morgan, 2007). It is a commonplace view that the thermodynamically irreversible nature of cyclin proteolysis underlies the unidirectionality o