Currently approved nucleoside reverse transcriptase inhibitors (NRTIs) include AZT (zidovudine, Retrovir), 3TC (lamivudine, Epivir), ddI (didanosine, Videx), abacavir (Ziagen), FTC (emtricitabine, Emtriva), and tenofovir (Viread). (Tenofovir is, strictly speaking, not a nucleoside analogue but a very closely related type of molecule called a nucleotide analogue). ddC (zalcitabine, Hivid) has been withdrawn.) Resistance to NRTIs is primarily caused by mutations in HIV's reverse transcriptase enzyme.