gp41 is a glycoprotein non-covalently bound to gp120, and provides the second step by which HIV enters the cell. It is originally buried within the viral envelope, but when gp120 binds to a CD4 receptor, gp120 changes its conformation causing gp41 to become exposed, where it can assist in fusion with the host cell.
The env gene does not actually code for gp120 and gp41, but for a precursor to both, gp160. During HIV reproduction, the host cell's own enzymes cleave gp160 into gp120 and gp41. See Replication cycle of HIV.
A polyclonal caprine antibody is in phase II human clinical trials that targets among others sites, the GP41 transmembrane glycoprotein. This is a new class of treatment for HIV infection. The trials are being conducted for for a patented biologic developed by Virionyx, as New Zealand Company. 
- "virionyx.com". Retrieved 2007-08-26.