Cell cycle control proteins expression. The expression of p53/pp53 and Chk2/pChk2, proteins possibly involved in G2 checkpoint, was measured by Western blotting in BEAS-2B cells exposed for 3 and 10 h to 7.5 μg/cm2 of winter PM2.5. The topoisomerase II inhibitor etoposide (Etop, 250 nM, 10 h exposure) was used as positive control for p53/pp53. Representative images of the Western blotting are shown and the results of 3 independent experiments are reported in the histograms as fold increase (FI) over the control values (mean ± SEM). *Statistically significant difference from untreated cells (control), P < 0.05.