- The genome length of human parainfluenza virus type 2 follows the rule of six, and recombinant viruses recovered from non-polyhexameric-length antigenomic cDNAs contain a biased distribution of correcting mutations. Skiadopoulos, M.H., Vogel, L., Riggs, J.M., Surman, S.R., Collins, P.L., Murphy, B.R. J. Virol. (2003)