Fig. 6. Stable overexpression of human Cdt1 in normal human fibroblasts activates the ATM-Chk2 DNA damage checkpoint pathway without rereplication. HFF2 cells were infected with the high-titer wild-type T7-Cdt1, the Cy mutant T7-Cdt1 or control retroviruses and selected. At 2 weeks after infection, cells were subjected to analyses. (A) Whole cell lysates were immunoblotted. (B) The signal intensities of the bands displayed in A were quantified, and shown with Cdt1wt set at 100. (C and D) Asynchronous (AS) cells or cells arrested in quiescence were treated with BrdU for 8 hours and then double-immunostained with anti-BrdU and anti-phosphorylated ATM antibodies.