Retroviruses are responsible for causing some forms of leukemia (a type of cancer) in humans, and the virus that causes AIDs (HIV) is a retrovirus. Despite this, retroviruses are also useful tools for protein engineering, used for introducing new genes into a host cell. These types of virus integrate into host DNA at random sites, however, therefore the locations for gene insertions cannot (as yet) be controlled.
Retroviruses are viruses with an RNA genome (RNA viruses), that replicate by way of reverse transcription of their RNA into DNA. These viruses penetrate their cell host intact. Once inside, their capsid protein coat is removed, releasing a single stranded RNA genome. An RNA-dependent DNA polymerase, encoded by the virus, generates a complementary DNA strand which becomes integrated into the host cell genome and serves as the template for the synthesis of new viral RNA genomes. Protein synthesis from these genomes results in development of new viral progeny which are released slowly from the host cell by budding, thereby not causing lysis and death of the infected cell, but allowing ongoing replication of new viruses.