Cell growth dilutes the cell cycle inhibitor Rb to trigger cell division

A cell size sensor Efforts continue to uncover the long-sought mechanism by which cells coordinate growth and cell division to maintain a constant size. Zatulovskiy et al. propose that human cells sense the concentration of the retinoblastoma family protein Rb. Rb itself acts on transcription factors to inhibit progression through the cell cycle. As cells grow, the concentration of Rb decreases by dilution, and the cell can then divide. In mice heterozygous or homozygous for an Rb gene deletion, the size of liver hepatocytes decreased—and variability in cell size increased—as each Rb allele was lost. Science , this issue p. 466