As the technology for gene therapy develops in vitro and in vivo in animal models, it is becoming clear that the three principal approaches--recombinant retroviruses, recombinant adenovirus, and direct DNA delivery--will ultimately have applications in specific therapeutic situations that take full advantage of the unique features of the specific delivery system: low level persistent expression after ex vivo recombinant retroviral therapy, high level transient expression after in vivo recombinant adenoviral therapy, or moderate level transient expression after in vivo administration of a synthetic DNA complex, which in principle could be repeated as desired.