Flexi Says: A patient's target cells, such as liver or lung cells are infected with the genetically altered virus. The vector then unloads its genetic material containing the therapeutic human gene into the target cell. The generation of a functional protein product from the therapeutic gene should restore the target cell to a normally functioning phenotype. To date, this process has had limited success, but further research should improve results. One of the issues is that the DNA ingested should be spread out to all the cells, and the cells are covered with mucus, so DNA has to try hard to get through the mucous barrier. DNA also has to be used for a longer period of time for spreading properly to each cell but the problem is that the body replaces the cells that line the lungs cells for example at every 48 hours. Thus, if you do cure the cells then after 48 hours new cells come up in which the DNA needs to be changed.