If a gene becomes damaged, this damage is called a mutation. Mutations can cause the gene to not work how it should and cells can grow out of control. When cells grow too quickly or uncontrollably, cancer forms. Things like smoking or the environment may cause these mutations, or they may be inherited. If we could fix these mutations, we could stop cancer from starting.<br />It is not easy getting the genes into the tiny cells. For this reason, a carrier (called a “vector”) is used. The most common vectors are viruses. The genes of the virus vector must be changed to carry human DNA. These viruses are like those that cause the common cold, only they are "deactivated" (turned off) so that they will not cause the patient to get the cold.<br />In some cases, cells are taken from the patient and mixed with the virus in a lab. The virus with the desired gene finds its way into the cells. These cells grow in the lab. The grown cells are then given back to the patient by intravenous (IV) infusion into a vein or are injected into a body part (such as the lung) or the tumor.<br />