Effect of ATBF1 on activation of p21 promoter in X-ray-irradiated ATM (+/+) and ATM (-/-) cells. ATM (+/+) and ATM (-/-) cells were transfected with p21 promoter-luciferase, pRL-TK-luciferase (as an internal control), and an indicated dose of HA-ATBF1 vector or pCI-HA vector as a control. After 24 h, cells were irradiated with a 2.5 Gy X-ray, and then luciferase activity was measured after 12 h. All the values are presented as the mean ± SEM of three independent experiments. **p < 0.01 vs not irradiated. N.S., not significant, as determined by Mann-Whitney U-test with Bonferroni Correction.