Virus infection: The growth of low pathogenic avian influenza virus in many cell lines requires the addition of trypsin to culture medium to ensure viral hemagglutination (HA) cleavage and multicycle infection. The replication kinetics of the H9N2 viruses was measured in the absence or presence of trypsin. In the absence of trypsin, H9N2 virus did not infect the HUVECs as detected by PFU. There fore,the trypsin was added to the culture mediumin all experiments on H9N2 virus infection.