Determination of growth kinetics and stability of the recombinant viruses in cells: Culture supernatants from cells transfected with infectious clones were harvested at 5 d post-transfection and designated 'passage (P) 1'. The P1 virus was used to inoculate fresh MARC-145 cells to collect P2 then P3 at an interval of 4-5 d between successive passages. Each passage virus was titrated, aliquoted and stored at −80 °C until use. Growth curves of the rescued viruses were evaluated by inoculating MARC-145 cells with P3 viruses at a MOI of 0.1. Aliquots of the supernatants of infected cells were collected at points with 10 h intervals until 100 h and the virus was titrated by determining log TCID50/mL to monitor growth kinetics [26] . To determine the stability of the recombinant virus, the rescued P1 virus was passaged on MARC-145 and PAM cells until P6. Expression of indicator proteins and IFN along with virus replication was detected by RT-PCR, western blotting and immunofluorescence as described above [15, 47] .