HIV-1 protease performs an essential step in the life cycle of HIV. Like many viruses, HIV makes many of its proteins in one long piece, with several proteins strung together. HIV-1 protease has the job of cutting this long "polyprotein" into the proper protein-sized pieces. The timing of this step is critical. The intact polyprotein is necessary early in the life cycle, when it assembles the immature form of the virus. Then, the polyprotein must be cut into the proper pieces to form the mature virus, which can then infect a new cell. The cleavage reactions must be timed perfectly, allowing the immature virus to assemble properly before the polyprotein is broken. Because of its sensitive and essential function, HIV-1 protease is an excellent target for drug therapy. Drugs bind tightly to the protease, blocking its action, and the virus perishes because it is unable to mature into its infectious form.