- Codon substitution mutations at two positions in the L polymerase protein of human parainfluenza virus type 1 yield viruses with a spectrum of attenuation in vivo and increased phenotypic stability in vitro. McAuliffe, J.M., Surman, S.R., Newman, J.T., Riggs, J.M., Collins, P.L., Murphy, B.R., Skiadopoulos, M.H. J. Virol. (2004)