The Jeryl Lynn (JL) vaccine against mumps virus (MuV) contains two components, MuV JL5 and MuV JL2, which differ by over 400 nt. Due to the occurrence of bias in the direction of mutation, these differences and those found in nucleotide sequences of different isolates of the minor component in the vaccine (MuV JL2) might be due to the effect of ADAR-like deaminases on MuV grown in tissue-cultured cells. A molecular clone of MuV JL2 (pMuV JL2) and MuV JL2-specific helper plasmids were constructed in order to investigate molecular interactions between MuV JL5 and MuV JL2, to augment the existing molecular clone of MuV JL5 (pMuV JL5) and MuV JL5-specific helper plasmids. Genome and mRNA termini of MuV JL2 were characterized, and an unusual oligo-G insertion transcriptional editing event was detected near the F mRNA polyadenylation site of MuV JL2, but not of MuV JL5. Genes encoding glycoproteins of rMuV JL2 and rMuV JL5 have been exchanged to characterize the oligo-G insertion, which associated with the specific sequence of the F gene of MuV JL2 and not with any other genes or the RNA-dependent RNA polymerase of strain MuV JL2. The results indicate that a single G-to-A sequence change obliterates the co-transcriptional editing of the F mRNA and that this oligo-G insertion does not affect the growth of the virus.