Illustration of places for application of criteria for synchronization. Numbers refer to criteria in list in Appendix 1 [see additional file 1]. The top box is an activity/cell graph, the lower box is a synchrony curve, and below the synchrony curve are DNA content (left) and size analyses (right) of synchronized cells. Note that the expression of a cycle-dependent gene should peak at the same part of the cell cycle in successive cell cycles. Also, the DNA content should progress as expected through the cell cycle and repeat in the second cycle. The cell size of synchronized cells should have a distribution that is significantly narrower than the unsynchronized, original culture.