The surface of the HIV virus contains a protein known as the envelope protein (Figure ), which plays an important role for viral entry into the cells. This protein exists in two conformations: open and closed. The envelope protein has two components known as gp120 and gp41. The gp120 protein contains a subprotein known as HT1 (shown in grey color in Figure ), which binds to the CD4 receptor (shown in orange color in Figure ) to gain entry into the cell. If the interaction between the viral protein (HT1) and human cell receptor (CD4) can be prevented, viral entry can be inhibited. An example of a medication exhibiting this mechanism of action is enfuvirtide, which belongs to a new class of antiretroviral therapies known as fusion inhibitors. It mimics the viral HT1 protein and binds to CD4 receptor, thus preventing viral entry into the cell.