The pCL-Ampho packaging vector is a part of the RetroMax expression system (Cat#10040K) 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 1 infectious particle/cell/day. Retrovirus tropism is determined at three 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. The plasmid DNA can be amplified by transforming bacteria and growing them on LB/ampicillin (100 ?g/ml) media.