Stabilizing the sequence of the rhMPV/ΔM2-2 genome by re-engineering all poly A tracts will only be partially effective because this does not address the increased frequency of point mutations. More studies are needed to gain detailed knowledge of the hMPV polymerase complex and the role of M2-2 during hMPV replication. Ablation of the M2-2 ORF also resulted in up-regulation of viral transcription but not genomic RNA and increased the frequency of readthrough transcription at the M2/SH noncoding region. Aberrant transcription regulation and increased genetic instability could both contribute to the attenuation phenotype rhMPV/ΔM2-2. Further studies are being conducted to develop a potential live hMPV vaccine candidate. [2, 17, 27] . The following recombinant viruses were generated by reverse genetics from full-length cDNA plasmids using the rhMPV/NL/1/00/ E93K/S101P backbone: rhMPV/ΔM2-2, rhMPV/GFP, rhMPV/GFPpolyA, rhMPV/ΔM2-2/GFP, and rhMPV/ ΔM2-2/GFPpolyA.