The nucleoside analog 2',3'-dideoxycytidine (ddCyd), also known as Zalcitabine is a nucleoside analog reverse transcriptase inhibitor (NRTI) sold under the trade name Hivid. HIVID is indicated in combination with antiretroviral agents for the treatment of HIV infection. It is used as part of a combination regimen with antiretroviral agents. But it was discontinued by Roche Pharmaceuticals on December 31, 2006 due to the availability of newer HIV medicines. Within cells, zalcitabine is converted to the active metabolite, dideoxycytidine 5'-triphosphate (ddCTP), by the sequential action of cellular enzymes. Dideoxycytidine 5'-triphosphate inhibits the activity of the HIV-reverse transcriptase both by competing for utilization of the natural substrate, deoxycytidine 5'-triphosphate (dCTP), and by its incorporation into viral DNA. The lack of a 3'- OH group in the incorporated nucleoside analogue prevents the formation of the 5' to 3' phosphodiester linkage essential for DNA chain elongation and, therefore, the viral DNA growth is terminated. The active metabolite, ddCTP, is also an inhibitor of cellular DNA polymerasebeta and mitochondrial DNA polymerase-gamma and has been reported to be incorporated into the DNA of cells in culture.