﻿﻿The proposed project will be a prospective, open-label treatment trial of 14 HIV-infected adults. Participants will be enrolled in two tracks. Track A will enroll participants who have been taking E/C/F/TDF to E/C/F/TAF for at least 12 weeks. Track B will enroll participants who are not taking ART and no prior exposure to elvitegravir and no genotypic drug resistance to any component of E/C/F/TDF to E/C/F/TAF. CSF and blood will be collected twice in Track A (two on-treatment assessments) and three times in Track B (one pre-treatment, two on-treatment assessments). If possible, we plan to time assessments to bridge the transition from E/C/F/TDF to E/C/F/TAF so that all participants will ideally have one CSF collection on each regimen, enabling a paired comparison of tenofovir concentrations between the two tenofovir formulations. We estimate that 28 on-treatment CSF collections will be sufficient to provide a small-sample estimate of the distribution of elvitegravir, tenofovir and TAF into CSF.