Which of the following represent obstacles to the use of retroviruses in therapeutic gene transfer?

a. Viral receptors may not be present on target cell membranes

b. For integration, the host cell must undergo mitosis

c. Virus particles are labile

d. Viral purification is difficult

There are several limitations of recombinant retroviruses which make them unsuitable for some gene transfer protocols. First, retrovirus entry requires that target cells contain the appropriate viral receptor. In many cases, these receptors are not known. Difficulties in efficiently transducing certain target cell populations may be due to low levels or absence of appropriate virus receptors. A second requirement for efficient retroviral gene integration is the process of cell proliferation. The dependence of integration on mitosis is thought to be due to the need for nuclear membrane breakdown to enable the viral integration complex to enter the nucleus. Production is another problem, as retrovirus particles are relatively labile when compared to other viruses. In general, retroviruses cannot be purified without significant loss of infectivity.