We thus interrogated the pseudovirus mutants for RT activity. The RT activity recovered from mutants Y271A and I274A were almost unnoticeable, as compared with that of wild type ( Fig. 2A) . This result was further confirmed using wild type, Y271A and I274A live viruses (data not shown). Next, we asked if the viral particles contain dysfunctional RT or the RT was not incorporated into the viral particles. By Western blotting, RT protein was basically undetectable in viral particles of these two mutants (Fig. 2B) . The results suggested that the mutations could affect the incorporation of RT into the viral particles, leading to a defect in reverse transcription which initiates viral replication.