Growth arrest abnormalities in p53-/- Clara cells a) 24 hour-BrdU incorporation assays demonstrating growth arrest abnormalities in irradiated p53-/- Clara cells. Clara cell cultures were incubated with BrdU from 26 to 50 hours in culture. Nuclei that had incorporated BrdU were detected with an anti-BrdU antibody (brown). b) The growth arrest response to ionizing radiation (IR) is entirely p53-dependent in Clara cells cultured in serum-free conditions. Addition of 5% foetal bovine serum (FBS) to the culture medium abolishes the p53-dependent DNA damage response. p53-/- cultures additionally display a higher rate of proliferation in the presence of serum, highlighting synergistic effects of p53 and extracellular factors in Clara cell cycle regulation. The data represents mean ± SD (n = 3), * indicates p < 0.05 (t test).