Sanguinarine transiently inhibits DNA synthesis. Control and drug-treated cells were pulsed with 10 μM BrdU for 30 minutes prior to fixation, and subsequently labeled with an antibody to BrdU (green) and counterstained with Hoechst (blue). (a and b) Note strong BrdU labeling of nuclei of S-phase cells. (c and d) Cells pulsed with BrdU 2 hours after the addition of 5 μM sanguinarine to the culture medium. Note the strong suppression of BrdU labeling. (e and f) Cells cultured in the presence of 5 μM sanguinarine for 24 hours prior to being pulsed with BrdU. Interestingly, a number of cells again display brightly labeled nuclei.