In 1987, four years after HIV was identified, the Food and Drug Administration (FDA) approved the use of azidothymidine (AZT) to slow the progression of HIV infection to full-blown AIDS. AZT targets reverse transcriptase, an enzyme essential to HIV replication in lymphocytes. Unfortunately, HIV evolves rapidly and develops resistance to AZT, making single-drug therapy with reverse transcriptase inhibitors ineffective. In 1996, a new class of antiretroviral drugs, called protease inhibitors, was approved. This development led to a treatment strategy called highly active antiretroviral therapy (HAART), a drug "cocktail" combining multiple drugs that target at least two steps in the HIV life cycle. As a result, deaths from AIDS in developed nations have dropped significantly, making HIV a chronic, treatable disease, not a death sentence. Since 1987, more than 20 drugs have been approved for use in combination to treat HIV infection, and more are in the pipeline. New drug classes include chemical agents that inhibit the virus's entry into the cell, integration into the host genome, and maturation.