Growth of rhMPV and rhMPV/ΔM2 in Vero cells Titer (log 10 PFU/ml) rhMPV/GFP, rhMPV/GFPpolyA, rhMPV/ΔM2-2/GFP, and rhMPV/ΔM2-2/GFPpolyA viruses. Titers ranged from 6.6 log 10 PFU/mL for rhMPV/ΔM2-2/GFP to 7.3 log 10 PFU/ml for rhMPV/GFPpolyA and plaque sizes between all four viruses were similar ( Figure 4A ). However, rhMPV/ΔM2-2 and rhMPV/GFP plaques were both smaller than rhMPV plaques.