ONCE - Only Nocturnal Combination Evaluation of Antiretroviral-Experienced HIV 1 Infected Subjects Switching to Atripla

This is a prospective study to evaluate pure virological response rates in antiretroviral-experienced HIV infected subjects initiating therapy with Atripla. Subjects will be switching to Atripla having already been established on the individual components of efavirenz, emtricitabine, and tenofovir DF. Within the Chelsea and Westminster hospital approximately 540 subjects have been identified who are currently receiving the individual components of Atripla and who would eventually switch to Atripla. The Royal Sussex County Hospital will be included to ensure that recruitment timelines are met. A minimum of 150 subjects will be switched within the first 6 months allowing initial 24 week data for these subjects to be available approximately 12 months post launch.