Drosophila Schneider 2 (S2) cells were purchased from Invitrogen Life Technologies and maintained in 75 cm flask (Corning) at the cell density between 0.2 to 2 × 10 6 cells per ml in the Express FIVE® SFM medium (Invitrogen Life Technologies) supplemented with 10% FBS, 100 U/ml penicillin, 100 U/ml streptomycin and 100 mg/L L-glutamine at 28°C without additional CO 2 .