Mutants of hMPV have been constructed by deleting M2-1, M2-2, SH or G, either individually or in combination, using the CAN97-83 isolate of hMPV, which requires trypsin for growth in cell culture [13, 14] . Recombinant hMPV lacking either M2-2 or G were attenuated and immunogenic in African green monkeys and have been proposed as promising vaccine candidates [15] . Such a suitably attenuated live hMPV is desirable because it would deliver the nearly complete set of viral antigens and closely mimic a natural hMPV infection.