P388 cells (A–D) or XS52 cells (E–F) were pre-treated with medium, or medium containing the indicated dose of imiquimod for twenty-four hours, and then irradiated with the 10 mJ/cm2 (A–B, E,F) or 25 mJ/cm2 (C,D) of UVB. After another twenty-four hours, DNA was extracted, denatured, and crosslinked onto a nylon membrane, and probed with a cyclobutane pyrimidine dimer specific monoclonal antibody. Antibody binding to the DNA was detected using a secondary antibody and chemiluminescence (See methods). DNA dot blots (A,C,E); densitometry scanning of DNA dot blots depicted as a line point graph (B,D,F).