Retroviruses are very efficient tools for delivering genes into dividing cells. The RetroMax™ Retroviral Expression vector system is based on the pCL vector system and has been designed to maximize recombinant–retrovirus titers in a simple, efficient, and flexible experimental system1. All members of the RetroMax expression vector family (pCLXSN, pCLNCX, pCLNRX, and pCLNDX) have an extended packaging signal and are derived from safety- modified retrovirus vectors in which the gag open reading frame has been stopped by a point mutation, thereby minimizing the opportunity for replication competent retrovirus production by recombination with packaging genome. Thee RetroMax system is designed for maximal virus titer in 293 cells. It takes advantages of two properties of 293 cells, i) high level of transfectibility, ii) strong E1A-mediated stimulation of CMV promoter controlled transcription. 293 cells are of nonmurine origin, hence the problem of selective packaging and transfer of VL30 genomes (present in all murine packaging cells) are avoided. Vector supernatants are free of helper virus and are of sufficiently high titer within 2 days of transient transfection in 293 cells to permit infection of more than 50% of dividing target cells in culture.