Separation of the cis and trans functions of a retrovirus in a recombinant, replication defective vector system. Replication defective retroviral vector systems are made by separating the cis (shown in black) and trans (shown in green) genetic functions of the virus into a vector construct, which contains the cis sequences, and helper or packaging plasmids, that encode the viral proteins (i.e. contain the trans sequences). To minimize overlap between the two components of the system heterologous transcriptional control elements (shown in red) are used to express the trans functions. Recombinant virus is made by introducing all these elements into the same cell. Only the vector transcript is incorporated into virions as this is the only RNA that contains the retroviral packaging signal (ψ).