Five subfamilies of aspartic proteases (EC 3.4.23.-) are classified, all sharing a highly conserved sequence of Asp-Thr-Gly. Compared to the three other types of proteases, serine, cysteine, and metalloproteases, aspartic proteases comprise a relatively small group. The aspartic proteases of many pathogens represent attractive targets for inhibitor design to control the progression of these diseases. The development of effective HIV protease inhibitor drugs for the treatment of HIV infection in AIDS illustrates the importance of this approach. Most of the aspartic proteases belong to a pepsin structural superfamily, having homologous primary and tertiary structures and nearly identical catalytic apparatus.