Various gag mutants of human immunodeficiency virus type 1 (HIV-1) generated in vitro were evaluated for their potentials to suppress the replication of wild-type (wt) virus. A single-round of wt virus replication in the presence of various mutant proteins was quantitatively monitored by transfection and infection experiments. Out of 38 mutants examined, 15 were demonstrated to interfere with the replication of wt HIV-1 at early and/or late viral replication phase. Some of these mutants were also effective against the replication of wt HIV-2. In this review, we focus on the mutants, which are able to act against a wide variety of HIV, and are very useful for future gene therapy against AIDS.