Budding yeast cells are assumed to trigger start and enter the cell cycle only after they attain acritical size set by external conditions. However, arguing against deterministic models of cell sizecontrol, cell volume at start displays great individual variability even under constant conditions.Here we show that cell size at start is robustly set at a single-cell level by the volume growthrate in G1, which explains the observed variability. We find that this growth-rate-dependentsizer is intimately hardwired into the start network and the Ydj1 chaperone is key for setting cellsize as a function of the individual growth rate. mathematical modelling and experimental dataindicate that a growth-rate-dependent sizer is sufficient to ensure size homeostasis and, as aremarkable advantage over a rigid sizer mechanism, it reduces noise in G1 length and providesan immediate solution for size adaptation to external conditions at a population level.
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