Mouse melanoma cell line B16 (obtained from China Center for Type Culture Collection, Wuhan University, Wuhan, China) was maintained in a complete RPMI-1640 medium supplemented with 10% fetal bovine serum and 1% Penicillin-Streptomycin. After a sublethal dose of X-ray radiation (18 Gy) using RS2000 X-ray irradiator (Rad Source Technologies Inc., GA, U.S.), 1×106 cells were subcultured in 25 cm2 plates in 5% CO2 at 37°C. Untreated cells cultured in the same conditions were used as control cells. Cells were harvested after 48 h culture for further experiments. The experiment was repeated 3 times.