The decision of whether and when individual cells commit to apoptosis is thought to be determined by many factors, including the molecular composition of single cells, environmental factors and the stochasticity of apoptotic pathways. Patrick Bhola and Sanford Simon (p. 4296) now provide new insight into this issue with their finding that apoptotic commitment is primarily determined by molecular composition, and that apoptotic variability within a population is dictated by the divergence of the individual cells that it comprises. As a measure of apoptosis, the authors use a novel single fluorophore reporter to measure the cleavage of a DEVD peptide by apoptotic proteases, and show that cell death in response to apoptosis-inducing drugs is locally synchronised in vitro. In this system, neither cell-cycle nor secreted factors have a significant effect on synchronising the timing of apoptosis in neighbouring cells. Rather, the authors show that sister cells undergo apoptosis with similar kinetics, and that the timing of apoptosis is more divergent among cells that are more distantly related. If protein expression is inhibited in newly divided cells, the timing of apoptosis among sister cells is preserved. The authors conclude, therefore, that the similar molecular composition of sister cells determines the synchrony of their commitment to apoptosis.