sentences:  Fusion inhibitor drugs such as enfuvirtide block the fusion process by binding to gp41.The Mouse Mammary Tumor Virus (MMTV) "env" gene codes for a polyprotein gp70 () that is cleaved to yield the surface (SU) and transmembrane (TM) Env products.Gp52 is the SU subunit in MMTV and gp36 is the TM subunit.