 The first nucleoside reverse-transcriptase inhibitor with in vitro anti-HIV activity was zidovudine.Since zidovudine was approved in 1987, six nucleosides and one nucleotide reverse-transcriptase inhibitor (NRTI) have been approved by FDA.NRTIs approved by the FDA are zidovudine, didanosine, zalcitabine, stavudine, lamivudine, abacavir and emtricitabine and the only nucleotide reverse-transcriptase inhibitor (NtRTI) approved is tenofovir (see table 4).