Transmembrane envelope protein of the human immunodeficiency virus which is encoded by the hiv env gene. It has a molecular weight of 41,000 and is glycosylated. The N-terminal part of gp41 is thought to be involved in cell fusion with the cd4 antigens of t4 lymphocytes, leading to syncytial formation. Gp41 is one of the most common hiv antigens detected by immunoblotting.