The effect of six RT mutations at the helix clamp turn in HIV-1 RT on viral replication was tested with pseudoviruses. When Jurkat cells were infected with the wild type and mutant pseudoviruses (250 ng p24 virus/5610 5 cells), viral replication was almost completely inhibited in the mutants Y271A and I274A, while replication of other mutants did not show significant difference as compared to that of the wild type (Fig. 1A) . A similar result was also obtained when the cells were infected with lower amount (150 ng p24) of pseudoviruses (data not shown). To confirm the above results, replication of wild type, Y271A and I274A live viruses was further monitored in U373-MAGI-CXCR4 CEM and MT2 cells. Mutants Y271A and I274A were undetectable in U373-MAGI-CXCR4 CEM cells (Fig. 1B) and no virus-specific CPE was found in MT-2 cells (Fig. 1C) . Since reverse transcription is the first step for HIV replication after viral entry into host cells, our results suggested that the two mutations might have affected the RT activity.