Viruses such as HIV that use a chemical called 'RNA' instead of DNA as their genetic material. Retroviruses convert their RNA genome into DNA and insert into their host's DNA chromosomes. This can disrupt genes and sometimes cause cancer. However, some retroviruses can be modified and used to deliver new genes into cancer cells in the laboratory. Scientists hope one day to use this technique, a type of gene therapy, to correct the genetic defects that cause cancer.