Abstract: A safe gene transfer system is provided based on simian immunodeficiency virus (SIV). A packaging construct is provided in which the SIV packaging sequence is removed, and, in one instance, the gag, pol and other accessory genes are expressed under the control of CMV promoter. SIV transfer constructs are provided in which cis-acting sequences required for viral replication (R, tRNA primer binding site, encapsidation site, RRE sequence and 3′ LTR), with or without a marker gene, are described. Stable packaging cell lines are provided that were prepared by transfecting 293(T) cells with an SIV packaging construct. High titer virus (>107 transducing units/ml) can be produced upon transient transfection with SIV transfer vectors. The safety of the described SIV gene transfer vectors was further tested in a primate model. Rhesus monkeys that received multiple inoculations of high titer (107) retroviral vectors did not develop any signs of side effects.