Virus stocks were obtained by using virus from a single plaque (ϳ10 7 PFU) to infect ϳ4 ϫ 10 6 cells to yield 6 ml of stock virus, which was then used to infect ϳ1 ϫ 10 7 cells at 33°C for 16 h to yield passage 2 virus stocks with a titer of ϳ5 ϫ 10 8 PFU/ml.