“…Furthermore, CD4 and p56 lck can be dissociated by agents such as phorbol esters (15,34) or HIV gp120 (4,16), leading to enhanced CD4 endocytosis. Finally, although fusion figures for HIV have been observed at the cell surface by electron microscopy (13,35,45), HIV virions are also frequently associated with endocytic clathrin-coated vesicles, and internalized virions can be observed in the process of fusion (1,13,30,35,37). Since only a small proportion of the virions in HIV preparations are actually infectious (22), it is unclear whether the viruses observed in these images would have gone on to establish productive infections.…”