Comparison of monocistronic and polycistronic viral transcripts showed differences in the frequency of readthrough transcription at the M2 gene end sequences between rhMPV and rhMPV/ΔM2-2 infected cells. In RNA from cells infected with rhMPV, the M2 riboprobes detected a minor monocistronic M2 transcript and a major polycistronic M2/SH readthrough transcript. While transcription readthrough is not unique to the M2/SH intergenic region, the polycistronic readthrough transcripts at other noncoding regions such as N/P and F/M2 were less pronounced and monocistronic transcripts predominated. The genes immediately upstream and downstream of the M2 and SH transcription units also existed predominantly as monocistronic transcripts indicating that the M2 gene-end sequences are particularly prone to high frequency of readthrough transcription. The frequency of readthrough transcription at the M2 gene stop sequences appeared to be accentuated by the removal of the M2-2 ORF. This may in part be attributed to the sequences that were removed and/or altered by the introduction of a Swa I site at the proximity of the M2 gene end sequences. Nonetheless, the increased frequency of readthrough at this gene junction may perturb the expression of downstream genes such as SH, G and L. In rhMPV/ ΔM2-2 infected cells, there are major populations of M2-1 transcripts that contained premature termination codons introduced by the high point mutation frequency. Therefore, it is possible that M2-1 expression was significantly reduced during rhMPV/ΔM2-2 infection and this reduction in M2-1 expression may also contribute to the up-regulation of transcription and increased frequency of read-through observed.