Studies of assembly, initially in a cell-free system and subsequently in cells, has revealed that newly synthesized HIV-1 Gag progresses through a series of post-translational assembly intermediates of increasing size (10S, 80S, 150S, and 500S intermediates) before producing the completely assembled 750S immature capsid. This pathway requires ATP at a discrete step as indicated and also requires the cellular ATPase ABCE1, which associates with selected assembly intermediates.