The relationship between cell size and cell division has been extensively studied in yeast. For some cells, there is a mechanism by which cell division is not initiated until a cell has reached a certain size. If the nutrient supply is restricted and the rate of increase in cell size is slowed, the time period between cell divisions is increased.

Yeast cell size mutants were isolated that begin cell division before reaching the normal size (wee mutants). The Wee1 protein is a tyrosine kinase. It normally phosphorylates the Cdc2 cell cycle regulatory protein (cyclin-dependent kinase-1, CDK1) on a tyrosine residue. This covalent modification of the molecular structure of Cdc2 inhibits the enzymatic activity of Cdc2 and prevents cell division.

In Wee1 mutants, there is less Wee1 activity and Cdc2 becomes active in smaller cells, causing cell division before the yeast cells reach their normal size. Cell division may be regulated in part by dilution of Wee1 protein in cells as they grow larger.