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Mini MCAT passage: Gene therapy using retroviruses

Problem

Several methods of gene therapy have been developed to insert foreign genes into cells with genetic defects. Microinjection of a gene into a target cell has been successful in some cases, but is very time consuming and requires a high degree of expertise. Another approach is electroporation, in which DNA is stimulated to enter cells by exposure to an electric shock. However, this procedure is traumatic to the cells. To date, the most effective method of introducing foreign genes into cells is through a viral vector. With this method, foreign genes enter the cell via a normal viral infection mechanism.
So far, retroviruses seem to be the most promising tool for gene therapy than either DNA viruses that integrate into the host cell or simple RNA viruses whose genomes are coded directly into protein. Retroviral vectors are constructed such that the therapeutic gene takes the place of gag, pol, or env coding regions that normally code for viral proteins. Problems associated with this form of gene therapy include the possibility that random integration could lead to activation of an oncogene, the fact that integration can occur only in cells that can divide, and the limitation that gene expression cannot be precisely controlled due to the randomness of integration.
Which of the following cells would not be a good target for gene therapy involving a retroviral vector?
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