The patients enrolled in the study accept a treatment regimen of RAL 400 mg twice daily plus OBT (optimized background therapy), which is selected on the basis of the patient's antiretroviral treatment history, results from all available genotypic resistance tests, previous or current laboratory abnormalities and intolerance to other antiretroviral drugs, and shall, if possible, contain two anti-retroviral drugs fully active against the patient's virus by genotype.