Several cyclins are responsible for regulating the cell cycle, but their regulatory plasticity makes their distinct roles difficult to define. On , Jacobs and colleagues discuss the particularly puzzling case of Cyclin A, which is essential for Drosophila embryonic viability but is needed only for certain types of mitoses. In the embryonic ectoderm, Cyclin A is only required for the very last division before cells become post-mitotic. The researchers have found that this is because during normal mitotic cycles, Cyclin A and Cyclin E function redundantly to prevent the premature activity of Fizzy-related/Cdh1 (Fzr), which targets the B-type Cyclins and String/Cdc25 for degradation. By contrast, before terminal mitoses, Cyclin E is inactivated early, leaving Cyclin A to work alone - this means that in Cyclin A mutants untimely Fzr activation prevents completion of the division programme. Observations from other labs, and those made in this paper, indicate that Cyclin A is also crucial for terminal mitoses in neuroblast lineages.
