HeLa is one of the oldest and most commonly used cell lines in biomedical 
research. Owing to the ease of which they can be effectively synchronized by 
various methods, HeLa cells have been used extensively for studying the cell 
cycle. Here, we describe several protocols for synchronizing HeLa cells from 
different phases of the cell cycle, including G1 phase using the HMG-CoA 
reductase inhibitor lovastatin, S phase with a double thymidine block procedure, 
and G2 phase with the CDK1 inhibitor RO-3306. Cells can also be enriched in 
mitosis using nocodazole and mechanical shake-off. Releasing the cells from 
these blocks enables researchers to follow gene expression and other events 
through the cell cycle. We also describe several protocols, including flow 
cytometry, BrdU labeling, immunoblotting, and time-lapse microscopy, for 
validating the synchrony of the cells and monitoring the progression of the cell 
cycle.