This study aims to analyze the association between the baseline detection of resistance mutations through Ultra deep Sequencing (UDS) and the virological outcome of salvage antiretroviral therapy, in comparison with conventional genotypic resistance tests. Based on the data generated in this study, new resistance interpretation tools and algorithms will be developed to improve the prediction of antiretroviral therapy outcomes. The final aim of the study is to improve the clinical care of HIV-1-infected patients through the incorporation of improved new antiretroviral resistances tests in the clinical practice.