Structure of the PIV genome in wild-type, attenuated, and chimeric viruses. (a) rPIV3-cp45 is a version of wild-type recombinant PIV3 (rPIV3 wt) that contains 15 point mutations (asterisks) that confer the ts and attenuation phenotypes of the biologically derived PIV3-cp45 vaccine candidate virus. (b) The HN and F ORFs of PIV1 wt virus were substituted for their counterparts in PIV3 wt, creating the chimeric rPIV3-1 virus. This was then attenuated by the further introduction of 12 of the 15 mutations of PIV3-cp45, resulting in rPIV3-1cp45, a live-attenuated vaccine candidate for PIV1.