MDBK cells which survived infection with a mutant of influenza virus A/WSN and subsequently received many passages contained viral M and NS genes in full length in the absence of virus production.

From a variant of MDBK cell line carrying the nucleotide sequences specific to a mutant of influenza virus A/WSN, we obtained cDNA clones representing viral M and NS genes in full length by polymerase chain reaction (PCR). The sequence analysis of five cDNA clones each for the respective genes revealed 4 to 10 base changes with M and 2 to 6 with NS compared with the corresponding genes of the original virus, although it was possible that at least some of them were ascribed to the artifacts during reverse transcription or Taq polymerase reaction.