In recent years, several novel treatment modalities emerged for a number of virus infections, including lamivudine for hepatitis B virus, abacavir, adefovir dipivoxyl and apropovir disprometil for human immunodeficiency virus, cidofovir for cytomegalovirus, and famciclovir (the oral prodrug of penciclovir) and cidofovir for other herpesviruses (i.e. herpes simplex virus and varicella-zoster virus). For all drugs, resistance eventually develops upon prolonged administration to the infected individuals, albeit at a varying extent. In addition, new mutations related to multidrug resistance have recently been identified.