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cDNAs encoding human parainfluenza virus type 4A and type 4B (hPIV-4A and-4B) fusion (F) proteins were cloned and sequenced. The predicted amino acid sequences of the F proteins had similar characteristic traits to those reported for the F proteins of other paramyxoviruses, They were more closely related to the F proteins of simian virus 5 (SVS), mumps virus (MuV), hPIV-2 and Newcastle disease virus (NDV) than to the F proteins of hPIV-1, hPIV-3, Sendai virus (SV) and measles virus (MV). In addition, hPIV-4A, hPIV-4B, SV5 and MuV shared a common feature of genomic organization: there was a small ORF between the F and haemagglutinin-neuraminidase (HN)-coding sequences, implying a common ancestry. Human parainfluenza viruses belong to the genu