Our results differ somewhat from that reported for the recombinant CAN97-83 strain of hMPV. Growth of recombinant rΔM2-2 CAN97-83 is trypsin-dependent and peak titer was not observed until 11 days post infection [14] . In contrast, our rhMPV/ΔM2-2 achieved peak titers at 4 days post-infection, a significant savings in production time. Interestingly, both ΔM2-2 viruses showed dramatic up-regulation of transcription at 48 hours post infection despite very different growth kinetics. No increase in the frequency of read-through transcription was observed for rΔM2-2 CAN97-83 whereas we observed increased polycistronic M2/SH transcripts in rhMPV/ ΔM2-2 infected cells. This may stem from differences in the construction of the M2-2 deletion. rΔM2-2CAN97-83 had a deletion of 152 nt in the M2-2 ORF whereas our construct had a deletion of 142 nt and a SwaI site introduced adjacent to the polyA tract of the M2 gene stop sequences. However, the ratio of polycistronic M2/SH transcripts to monocistronic M2 transcripts was significantly different even between the two wild-type hMPV strains, with the Netherlands strain exhibiting a higher frequency of readthrough at the M2/SH noncoding region than the Canadian strain.