sentences:  To form an active fusion protein, SU gp120 and TM gp41 polypeptides remain non-covalently bound together, but this interaction is often not stable, leading to shed, soluble gp120 and membrane-bound, gp41 'stumps'.Separately, cleavage by furin is inefficient, and virions often are released with inactive, uncleaved gp160.Because of the high prevalence of these inactive forms, the immune system often produces antibodies which target inactive Env, rather than active forms of the envelope protein.