The pCL-ECO packaging vector is a part of the RetroMax expression system and has been designed to maximize recombinant-retrovirus titers in a simple, efficient, and flexible experimental system. By introducing a retroviral vector into a cell expressing retroviral proteins, retroviral particles (virions) are shed into the culture medium at the rate of about 1 infectious particle/cell/day. Retrovirus tropism is determined at 3 levels. The first is simply a function of viral envelope protein, gp70. The envelope determines which cells the virus will enter. gp70 comes in three different flavors for gene therapy. Retroviruses obtained by cotransfection with pCL-Eco vector will infect mouse and rat cells, but not human cells.