Reverse-transcriptase inhibitors (RTIs) are a class of antiretroviral drugs that inhibit the activity of reverse transcriptase, a viral DNA polymerase that is required for replication of HIV and other retroviruses. Three forms of RTIs are known, of which nucleoside- and nucleotide reverse transcriptase inhibitors (NRTIs and NtRTIs respectively) essentially show similar modes of action, while non-nucleoside reverse-transcriptase inhibitors have a completely different mode of action. NNRTIs block reverse transcriptase by binding at a different site on the enzyme, compared to NRTIs and NtRTIs. NNRTIs are not incorporated into the viral DNAbut instead inhibit the movement of protein domains of reverse transcriptase that are needed to carry out the process of DNA synthesis. NNRTIs are therefore classified as non-competitive inhibitors of reverse transcriptase.