Protocol uses the plant amino acid mimosine as a G1/S synchronizing agent. Cells are first treated with excess thymidine to accumulate the majority of them at G1/S; however, some cells will have stopped growth within the S phase. Thymidine is then removed to allow all the cells to proceed completely through the S phase. Mimosine is then added to arrest the cells at the G1/S border. When mimosine is removed, cells will begin to enter S phase within about 1 hour.