The majority of current antiviral drugs target viral genome replication (eg, reverse transcriptase inhibitors) or virion maturation (eg, protease inhibitors). Although efficient, these drugs suffer from the quick appearance of drug-resistant mutants and usually do not eliminate the virus completely. Combination therapy may delay but not eliminate the emerging resistant mutants. The continuing search for novel antiviral compounds has been extended to earlier stages of viral replication cycle, such as virus entry. One particular group of entry inhibitors, the fusion inhibitors, interferes with fusion of viral and host cell membranes, a post-attachment process specific for enveloped viruses.