Using reverse genetics, we engineered rhMPV lacking the M2-2 gene with the aim of generating a potential vaccine candidate. rhMPV/ΔM2-2 grew to high titer in Vero cells, was attenuated in the respiratory tract of hamsters, and protected immunized hamsters from challenge with wthMPV/NL/1/00. These results agree with a similar study reported by Buchholz et al. in which a different subtype A hMPV strain, CAN97-83, with a deletion of M2-2 was proposed as a potential vaccine candidate [14, 15] . Our studies utilized the rhMPV/NL/1/00/E93K/S101P backbone which contained engineered mutations in the hMPV F gene that allows this virus to replicate efficiently in Vero cells without trypsin supplementation [17] . This property is expected to facilitate the testing and manufacture of potential live hMPV vaccine candidates. To assess the genetic stability of our M2-2 deletion mutant, sequence analyses were performed on P4 stocks of rhMPV/ΔM2-2. These analyses revealed major subpopulations (as high as 50%) that contained insertions of A nucleotides (sense direction) in the M2-1 and SH ORFs. These insertions appeared predominantly in A tracts and were also observed in non-hMPV sequences. Nucleotide insertions were also readily detected in an A tract introduced in the GFP ORF. Interestingly, insertions of A were not observed outside the region encompassing the nonessential genes M2 and SH. Transcriptional editing, whereby alternative reading frames of viral genes are accessed, has been observed in the P gene of several paramyxoviruses [19] [20] [21] [22] [23] . Therefore it is possible that an inserted A could occur frequently during transcriptional editing of paramyxovirus RNA. The nucleotide insertions observed in rhMPV M2-2 deletion mutants differ somewhat from transcriptional editing in that (i) the positions of inserted A nucleotides did not appear to be sequence biased beyond selecting for A tracts and is not hMPV sequence specific, and (ii) the nucleotide insertions were incorporated into the viral genome and could be propagated, as shown by passaging of fluorescent rhMPV/ΔM2-2/GFPpolyA viruses. Interestingly, these insertions did not appear to confer growth advantages in Vero cells because further passaging of rhMPV/ΔM2-2 promoted new A insertions and did not increase the subpopulations of earlier insertions. Many of the A nucleotide insertions caused premature translation terminations in the non-essential M2-1 and/or SH ORFs. These observations argue mechanistically against transcriptional editing and suggest that the insertions observed when M2-2 was deleted may be caused by an alteration in the fidelity of the replication complex directly or indirectly.