Effects of PPD on colorectal cancer cell cycle. HCT-116 and SW-480 cells were treated with 10–30 μM PPD for 48 h, then fixed in ethanol and stained with propidium iodide. DNA content was determined by flow cytometry. (A) Representative histograms of DNA content in each experimental group. (B) Percentage of each cell cycle phase with various treatments. Data are presented as the mean ± standard error. *P < 0.05, **P < 0.01 vs. control.