 Highly active antiretroviral therapy HAART successfully controls HIV-1 replication in most individuals, resulting in substantial immune restoration and decreased morbidity and mortality. However HIV-1 cannot be eradicated from infected individuals by current regimens. Cellular reservoirs and tissue sanctuary sites allow latent HIV-1 persistence and ongoing low-level virus replication, despite maximum virus suppression on HAART. This project aims to characterize the residual replication of HIV-1 in subjects on HAART, particularly in the gut-associated lymphoid tissue GALT. A group of 20 HIV-1-infected patients on effective HAART will undergo GI endoscopy and GALT biopsies will be taken. A sample of venous blood will also be collected. These samples will be use to characterize the residual viral populations on effective HAART in three compartments, plasma, monocytes, and GALT. HIV-1 coreceptor usage and its evolution on HAART will be characterized in virus reservoirs. This project could provide further insights into the residual replication of HIV-1 in subjects receiving HAART. 