 To achieve efficient inhibition of HIV-1 replication in patients, and to delay or prevent appearance of drug resistant viruses, drug combinations are used.HAART, also known as highly active antiretroviral therapy consists of combinations of antiviral drugs which include NRTIs, NtRTI, non-nucleoside reverse-transcriptase inhibitors and protease inhibitors.Currently, there are several NRTIs in various stages of clinical and preclinical development.