To assess the frequency of insertions that restored expression of GFP, Vero cells in 96-well plates were inoculated with P2 stocks of rhMPV/ΔM2-2/GFPpolyA or rhMPV/ GFPpoly A. GFP expression was monitored by fluorescence microscopy 4 days post infection. Plates were inoculated with 1, 10, 100 or 1000 PFU/well (Table 1) . No GFP-expressing foci were observed in wells inoculated with either 100 or 1000 PFU/well of rhMPV/GFPpoly A (Table 1 ). In contrast, cells inoculated with 10, 100, or 1000 PFU/well of rhMPV/ΔM2-2/GFPpoly A developed fluorescent foci. Fluorescent multicellular foci were observed in 25 out of 384 wells (6%) inoculated with 10 PFU/well of rhMPV/ΔM2-2/GFPpolyA (Table 1) . At 100 PFU/well of rhMPV/ΔM2-2/GFPpolyA, fluorescence was observed in 65% of the infected wells (Table 1) . Finally, fluorescent multicellular foci were observed in 100% of wells inoculated with 1000 PFU/well of rhMPV/ΔM2-2/ GFPpolyA. Thus, this assay shows that at least one insertion occurs out of approximately every 17 infections at 10 PFU/infection and the frequency of insertions was significantly elevated in the absence of M2-2.