One of the most challenging aspects of gene therapy is inserting genes into cancer cells and experts are striving to find new and improved techniques for achieving this. One of the main ways this is carried out is through the use of a vector, which carries a gene into a cancer cell. Usually, the vector is a virus because viruses are built to target and enter cells so they can deliver the their genetic material once inside them. Scientists have found ways to alter these viruses so that they only deliver genes to cancer cells rather than healthy cells. Other vectors are also being tested such as inactivated bacteria.