In the Golgi, cleavage of the retroviral Env precursor occurs at a polybasic (e.g., K/R-X-K/R-R) motif by cellular proteases such as furin or closely related enzymes probably within or near the trans-Golgi network (TGN) [37] [38] [39] [40] [41] [42] [43] . For HIV-1, the surface glycoprotein gp120 and the TM glycoprotein gp41, which bind together noncovalently, are both formed from the same precursor protein, gp160. Gp160 processing is essential for the activation of Env fusogenicity and virus infectivity [38, 42, [44] [45] [46] . Similarly, cleavage of Env is also essential for membrane fusion and virus infectivity in MLV [39, [47] [48] [49] [50] , in RSV [51, 52] , and in mouse mammary tumor virus (MMTV) [53] . A recent report showed that cleavage of MLV Env by furin also plays an important role in Env intracellular trafficking and incorporation [54] . Although most retroviral Env proteins including that of HIV-1 are associated with intracellular membranes [55] [56] [57] , at least part of the gp120/gp41 trimer complex traffics through the secretory pathway to the PM. It has been suggested that AP-1, one of adaptor proteins for clathrin-coated vesicle formation, is involved in the correct sorting of HIV-1 Env from the TGN to the PM, [58, 59] . It has been reported that intracellular CTLA-4-containing secretory granules are involved in the trafficking of HIV-1 Env to the PM although the subsequent trafficking of Env after the Golgi is not well understood [60] .