Growing without a size checkpoint

for granted that they are no longer challenged, despite a lack of experimental data to support them. Researchers who bravely challenge such widely accepted views often encounter resistance from others in the field. In this issue of the Journal of Biology [1], Ian Conlon and Martin Raff describe a series of experiments that questions a basic assumption about the way that mammalian cell size is maintained during proliferation, and they demolish widely accepted doctrine (see ‘The bottom line’ box for a summary of their work). Most proliferating cells in culture maintain a constant distribution of sizes and a constant average size, presumably by coordinating cell growth with progression through the cell division cycle. But it is far from clear how this coordination is orchestrated at the molecular level. Raff was perplexed that such a fundamental issue has attracted relatively little attention over the last couple of decades: the impressive advances in understanding the mechanisms of cellcycle progression have vastly overshadowed any insights gained into how cells coordinate their growth with their size. “It’s quite inexplicable,” says Raff, “why this question has been so neglected.” Yeast cell biologist Doug Kellogg (University of California Santa Cruz, USA) shares Raff’s amazement. “We really don’t know very much about this fundamental issue,” says Kellogg. “It’s one of the last big unsolved problems in cell biology.”