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efficient type of virus called a lentivirus, which could infect non-dividing cells, means that retrovirii have been superceded before they were ever really a candidate.