use of recombinant retroviruses was pioneered by Richard Mulligan
and David Baltimore with the Psi-2 lines and analogous retrovirus
packaging systems, based on NIH 3T3 cells (10, 11). Such helper-defective
packaging lines are capable of producing all the necessary trans
proteins--gag, pol, and env-- that are required for packaging,
processing, reverse transcription, and integration of recombinant
genomes. Those RNA molecules that have in cis the Psi packaging
signal are packaged into maturing virions.