Envelope glycoprotein GP120 (or gp120) is a glycoprotein exposed on the surface of the HIV envelope. gp120 was discovered by Professors Tun-Hou Lee and Myron"Max" Essex of the Harvard School of Public Health in 1988. The 120 in its name comes from its molecular weight of 120 kDa. Gp120 is essential for virus entry into cells as it plays a vital role in attachment to specific cell surface receptors. These receptors are DC-SIGN, Heparan Sulfate Proteoglycan and a specific interaction with the CD4 receptor, particularly on helper T-cells. Binding to CD4 induces the start of a cascade of conformational changes in gp120 and gp41 that lead to the fusion of the viral with the host cell membrane. Binding to CD4 is mainly electrostatic although there are van der Waals interactions and hydrogen bonds.Env is a viral gene that encodes the protein forming the viral envelope. The expression of the env gene enables retroviruses to target and attach to specific cell types, and to infiltrate the target cell membrane.Env is a viral gene that encodes the protein forming the viral envelope. The expression of the env gene enables retroviruses to target and attach to specific cell types, and to infiltrate the target cell membrane.