This study will be a retrospective observational study assessing identifiable tolerability problems in terms of their comparative incidence among Human Immunodeficiency Virus (HIV) patients treated with cART regimens including one of four different protease inhibitors [Atazanavir sulfate (ATV), Darunavir (DRV), Fosamprenavir (FPV), or Lopinavir (LPV)] and the impact of these identifiable tolerability problems on PI persistence, healthcare utilization, and healthcare costs.