Recombinant viruses were generated from cDNA as described previously [27] . Briefly, 1.2 ug of pCITE hMPV N, 1.2 ug of pCITE hMPV P, 0.9 ug of pCITE hMPV M2, 0.6 ug pCITE hMPV L, and 5 ug of full-length hMPV cDNA plasmid in 500 uL optiMEM containing 10 uL lipofectamine 2000 (Invitrogen), was applied to a monolayer of 10 6 BSR/T7 cells. The medium was replaced with fresh optiMEM 15 hr post transfection and incubated at 35°C for 2 to 3 days. Cells and supernatant from the transfection were harvested together and used to infect Vero cells. Virus recovery was verified by positive immunostaining 6 days post inoculation with ferret polyclonal Ab directed to hMPV. Recovered viruses were further amplified in Vero cells by inoculating at a multiplicity of infection (MOI) of 0.1 PFU/cell.