The routine protocols for cpe, plaque assay and growth curve experiments were described in detail previously [25, 28, 29, 41] . Briefly, for cpe and growth curve experiments PS cells in 24-well plates were infected with viruses at a multiplicity of infection (moi) of 1 PFU/cell, in four replicates. To estimate cpe, the inoculum was replaced with RPMI medium after virus adsorption for 1 h at 37C. Infected cell monolayers were initially examined by microscopy and then stained with 0.1%naphthalene black at 24, 48, 72, 96 or 120 h post-infection for further examination of the extent of cpe. For plaque assay, original virus stocks were 10-fold serially diluted and after 1-h of virus adsorption at 37uC infected monolayers were overlaid with 1% SeaPlaque Agarose (Cambrex, USA). After incubation at 37uC for 5 days monolayers were fixed with 10% formol saline and stained with 0.1% naphthalene black. For virus growth curve experiments, after virus adsorption for 1-h at 37uC, monolayers were washed 5 times with serum-free RPMI medium and overlaid with 1 ml of medium containing 2% FCS. The supernatant medium from infected cells was collected at 2, 10, 12, 16, 20 and 24 hours pi and frozen at 270uC. The titres of infectious virus were determined by plaque assay.