Effect of hydroxyurea on DNA synthesis in Jurkat cells. Jurkat cells were then treated with either vehicle (A) or 10 mM hydroxyurea (B), a DNA synthesis inhibitor, for six hours. Cells were then washed and further incubated for 12 hours followed by fixation, staining, and cytometric analysis. Brief inhibition of DNA synthesis with 10 mM hydroxyurea results in cell cycle synchronization. When the hydroxyurea is removed from the medium, the cells re-enter the cell cycle together, resulting in a large S-phase (DNA synthesis) peak 12 hours later as shown in panel B.