In addition to the circumstantial evidence discussed above, some biochemical data suggest a direct interaction between Gag and Env. In vitro binding between MA and a gp41 CT-GST fusion protein has been reported for both HIV-1 and SIV [112, 113] . Peptides corresponding to a large central domain of gp41 CT inhibited the capture of membrane-free Pr55 Gag with an anti-p24 antibody [114] . In addition, a stable, detergent-resistant gp41-Pr55 Gag interaction was detected in immature HIV-1 virions. The retention of gp41 in detergent-treated virions is dependent on the CT region, suggesting a direct or indirect interaction between Pr55 Gag and gp41 [115, 116] .