Specific siRNA targeting DNA-PKcs was designed and provided by Ribobio (Cat#: siG0812181317001 and siG0812181317002). Transfection of siRNA was done according to the manufacturer's protocol. Briefly, cells were plated in 60-mm culture plates at 5×105 cells per well, grown for 12 h, then transfected with siRNA using Lipofectamine 2000 (Invitrogen). Transfected cells were incubated at 37°C with 5% CO2 for 24 h.