HCT 116 colon adenocarcinoma cell line (p53+/+) were obtained from ATCC (Manassas, VA); HCT 116 p53−/− cells were kindly provided by Dr. Bert Vogelstein [10 (link), 11 (link)]. Cells were cultured in RPMI-1640 medium with L-glutamine (BioWhittaker Inc., Walkersville, MD), containing 10% heat-inactivated fetal bovine serum and 100 units of penicillin/mL and 100 µg/mL of streptomycin (Biofluids, BioSource, Rockville, MD) in an incubator with 95% air, 5% CO2, and 95% humidity at 37°C.