Viruses and cells
The H9N2 virus F/98 was isolated in Shanghai in 1998, stored at - 70 °C at the Animal Infectious Disease Laboratory, School of Veterinary Medicine, Yangzhou University. The GenBank accession numbers of the sequence of the F/98 strain are AY253750‑AY253756 and AF461532 (Lu et al., 2005). Human embryonic kidney cells (293T) and Madin‑Darby canine kidney (MDCK) cells, purchased from ATCC (Manassas, VA, USA), were maintained in Dulbecco’s modified Eagle’s medium (DMEM) (Sigma, St. Louis, MO, USA) supplemented with 10% fetal calf serum (Hyclone, South Logan, UT, USA) and were incubated at 37 °C with 5% CO2.