T-20 is the first drug to be developed which specifically inhibits the function of the gp41 transmembrane glycoprotein. By inhibiting the essential protein-protein surface interaction, T-20 is able to block the process of virus-to-host cell membrane fusion. Combination antiretroviral regimens (reverse transcriptase inhibitors plus PIs) have benefited many HIV patients, but heavily pretreated patients often develop multi-drug resistance via multiple gene mutations. A pharmacologic agent, such as T-20, that is effective at an alternative point in the virus replication cycle will make a valuable addition to the treatment of HIV infection.