Cell survival was measured by MTS assay (Promega Inc, Madison, Wis). Cells were seeded onto 96-well plates at 104 cells/well in Dulbecco Modified Eagle Medium with 10% fetal calf serum. Peptides were added to the media the next day, and spectrophotometric assay was performed 24 hours later. TUNEL (terminal deoxynucleotidyl transferase–mediated fluorescein-dUTP nick-end labeling) assay was performed 8 hours after addition of the indicated peptides using the Apoptag kit (Intergen Inc, Purchase, NY). The broad-spectrum caspase inhibitor Z-Val-Ala-Asp(OCH3)-fluoromethylketone (BIOMOL Research Laboratories Inc, Plymouth Meeting, Pa) was added to MTS plates as indicated. For Western blot analysis and reverse transcription–polymerase chain reaction (RT-PCR), cells were obtained at various points after the addition of 200µM Tat-αHDM2, lysates were obtained, and protein concentration was normalized. Western blot analyses were performed using a polyclonal p53 antibody (sc-6243; Santa Cruz Biotechnology Inc, Santa Cruz, Calif), dilution 1:500, and a polyclonal p73 antibody (sc-7957; Santa Cruz Biotechnology), 1:500 dilution. For semiquantitative RT-PCR, total RNA was obtained using the RNEasy kit (Qiagen, Valencia, Calif). Reverse transcription was performed in 20 µL of RNase-free water containing 5mM magnesium chloride, 10mM Tris-hydrochloride (pH 8.8), 50mM potassium chloride, 0.1% Triton X-100, 1mM dNTP, 20 U of rRNase inhibitor, 15 U of AMV reverse transcriptase (Promega Inc), and 0.5 µg oligo(dT)15 nucleotide primers at 42°C for 60 minutes, 99°C for 5 minutes, and then 5°C for 5 minutes. For PCR, 5 µL of the reverse transcription product was added to a 25-µL mixture containing primers for p21, Bax, Pig3, or GAPDH, and 2.5 U of Taq polymerase in standard PCR buffer (GIBCO BRL, Grand Island, NY). Reaction cycles (n = 20-25) and temperatures varied for each gene (details and primer sequences are available from the authors on request).