In 1964 zidovudine (AZT) was synthesized by Horwitz at the Michigan Cancer Foundation. The 3´hydroxyl group in the deoxyribose ring of thymidine is replaced by an azido group which gives us zidovudine. The lack of the 3´hydroxyl group which provides the attachment point for the next nucleotide in the growing DNA chain during the reverse transcription  makes it an obligate chain terminator. Ziduvodine is incorporated in place of thymidine and is an extremely potent inhibitor of HIV replication. This compound had been prepared in 1964 as a potential anti-cancer agent but was shown to be ineffective. In 1974 zidovudine was reported to have activity against retroviruses and was subsequently re-screened as an antiviral when the AIDS epidemic hit Western societies during the mid-1980s. However, zidovudine is relatively toxic since it is converted into the triphosphate by the cellular enzymes and therefore it is activated in uninfected cells.