A virus containing RNA, which infects by copying its own RNA into the DNA of the target cell. HIV, and some tumor-causing virii are retroviruses.

Engineered retrovirii also have great potential for curing genetic diseases, and possibly other viral diseases. Their most important property is that they insert their genetic material into the host genome, where it will remain forever. All descendents of this cell will also contain the same genes. For example, a custom retrovirus could contain a special gene for insulin, and only infect cells in the pancreas that normally secrete insulin. They could then insert a functional copy of the insulin gene into the genome, and the cell would begin producing insulin. This would cure a few types of diabetes (in theory)

Retrovirii are a type of virus that uses RNA instead of DNA to store its genetic information. When it enters a cell, it uses the cell's ribosomes to produce reverse transcriptase, which converts RNA to DNA and inserts itself into the cell genome where it is copied and creates many more of itself.

For several years it was thought that retrovirii were the future for gene therapy because of their consistent integration into the human genome, meaning that the virus could be administered once for a permenant cure. However, the fact that they can only infect dividing cells, and the discovery of a more efficient type of virus called a lentivirus, which could infect non-dividing cells, means that retrovirii have been superceded before they were ever really a candidate.