Analysis of S phase events requires the ability to synchronize the cell cycle for subsequent physiological or molecular analysis. The cell cycle can be arrested at different stages, using drugs or temperature-sensitive mutations, and then released (block and release). These methods offer high levels of synchrony. Synchrony is lower in methods that fractionate the cells (lactose gradient synchrony), which avoids any cell-cycle perturbation. The degree of synchronization is assessed by monitoring cell morphology or DNA content. The choice of method is influenced by the needs of the experiment and the requirements of any mutations already in the strain of interest.