unexpectedly, Cln3 is not the answer...

Cell growth dilutes a cell cycle inhibitor, Whi5, while the cell cycle activator, Cln3, remains at constant concentration. When Whi5 is diluted below a critical concentration, cells enter the division cycle. Dilution of a cell cycle inhibitor is a new concept in cell biology and an elegant solution to the problem of coupling growth to division.

Read all about it here http://www.nature.com/nature/journal/vaop/ncurrent/full/nature14908.html?WT.ec_id=NATURE-20150924&spMailingID=49614214&spUserID=NDk5Njc4MjI2OTYS1&spJobID=763182077&spReportId=NzYzMTgyMDc3S0

AuthorJan Skotheim