Tifuvirtide (also known as T 1249) is a synthetically designed hybrid retroviral envelope polypeptide patented by Trimeris, Inc. as fusion inhibitor useful for the treatment of HIV infection. Tifuvirtide binds to HIV glycoprotein 41 (gp41) and prevents its fusogenic conformation, inhibiting viral entry into host cells. Tifuvirtide is composed of sequences from HIV-1, HIV-2, and simian immunodeficiency virus. Tifuvirtide has greater potency in vitro, compared with enfuvirtide, and elicits activity against most enfuvirtide-resistant isolates in vitro and in vivo. A successful short-term phase 1/2 studies evaluation of antiretroviral activity and safety in humans proved the potential of this new drug, although further clinical development was discontinued.