cis elements required for the encapsidation of human immunodeficiency virus type 1 (HIV1) RNA have been investigated by using a replication-competent helper virus to package a series of HIV-i-based vectors which had been stably transfected into human CD4 T-cell lines. A previously identified packaging signal in the 5' leader region was not sufficient for the encapsidation of small vectors containing heterologous genes. In contrast, vectors containing additional gag and env sequences were

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... d with high efficiency and transduced into CD4-expressing target cells with titers exceeding 104 CFU/ml. The presence ofgag sequences did not enhance vector packaging efficiency. A 1.1-kb env gene fragment encompassing the Rev-responsive element was 3997