Sequence analysis of rhMPV CAN 97-83, showed that mutations do develop in SH, G, L and non-coding regions (NCR), with a particularly high frequency in the SH gene [26] . While mutations and insertions were reported for rhMPV and rhMPVΔ G of the CAN 97-83 strain, the sequence analysis did not include viruses that lacked the M2-2 gene [26] . In a separate evaluation of rΔM2-2 CAN97-83, no increase in the frequency of point mutations was reported [14] . While the M2-2 proteins of both strains are completely identical, the SH protein of the CAN97-83 strain is only 83% identical to the NL/1/00 strain. There are also 26 amino acid differences in the L gene between the two strains. Finally, although deletion of the hMPV M2-2 ORF succeeded in attenuating both hMPV strains, the CAN97-83ΔM2-2 virus was more attenuated in hamsters than rhMPV/ΔM2-2 [14] . Clearly there are differences between the CAN97-83 and hMPV/NL/1/ 00 strains. Further study will be required to elucidate the differences in phenotype.