ACTIVITY REGULATION Protease: The viral protease is inhibited by many synthetic protease inhibitors (PIs), such as amprenavir, atazanavir, indinavir, loprinavir, nelfinavir, ritonavir and saquinavir. Use of protease inhibitors in tritherapy regimens permit more ambitious therapeutic strategies. Reverse transcriptase/ribonuclease H: RT can be inhibited either by nucleoside RT inhibitors (NRTIs) or by non nucleoside RT inhibitors (NNRTIs). NRTIs act as chain terminators, whereas NNRTIs inhibit DNA polymerization by binding a small hydrophobic pocket near the RT active site and inducing an allosteric change in this region. Classical NRTIs are abacavir, adefovir (PMEA), didanosine (ddI), lamivudine (3TC), stavudine (d4T), tenofovir (PMPA), zalcitabine (ddC), and zidovudine (AZT). Classical NNRTIs are atevirdine (BHAP U-87201E), delavirdine, efavirenz (DMP-266), emivirine (I-EBU), and nevirapine (BI-RG-587). The tritherapies used as a basic effective treatment of AIDS associate two NRTIs and one NNRTI.MISCELLANEOUS HIV-1 lineages are divided in three main groups, M (for Major), O (for Outlier), and N (for New, or Non-M, Non-O). The vast majority of strains found worldwide belong to the group M. Group O seems to be endemic to and largely confined to Cameroon and neighboring countries in West Central Africa, where these viruses represent a small minority of HIV-1 strains. The group N is represented by a limited number of isolates from Cameroonian persons. The group M is further subdivided in 9 clades or subtypes (A to D, F to H, J and K).MISCELLANEOUS Resistance to inhibitors associated with mutations are observed both in viral protease and in reverse transcriptase. Most of the time, single mutations confer only a modest reduction in drug susceptibility. Combination of several mutations is usually required to develop a high-level drug resistance. These mutations are predominantly found in clade B viruses and not in other genotypes. They are listed in this entry which is a representative of clade B.