 Once incorporated into the DNA chain, the absence of a 3´-hydroxyl group, which normally forms the 5´- to 3´- phosphoester bond with the next nucleic acid, blocks further extension of the DNA by RT, and they act as chain terminators.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.