Hypersensitivity reactions have been observed with a number of different antiretroviral drugs, notably abacavir (Ziagen), the only nucleoside reverse transcriptase inhibitor known to cause the condition. Non-nucleoside reverse transcriptase inhibitors can also cause hypersensitivity reactions, although less frequently, while some protease inhibitors and the fusion inhibitor T-20 (enfuvirtide, Fuzeon) can produce similar symptoms, although they may not be caused by a true hypersensitivity.