Reverse transcription on purified viral RNA was performed using the Superscript VILO cDNA synthesis kit (Invitrogen) according to protocol. Briefly, 14 ml of extracted viral RNA was put into a 20 ml-reaction containing Superscript enzyme and random primers, and incubated at 42uC for 1 hour. The enzyme was inactivated at 95uC for 5 minutes and the sample stored at 220uC or immediately used.