3D reconstruction of NL4-3 and NL4-3/ADA gp140 trimers. Density maps of NL4-3 gp140 trimer (grey) and NL4-3/ADA gp140 trimer (cyan) viewed along (A, top view) and perpendicularly to its threefold symmetry axis (B, side view). Both density maps show a trimeric propeller-like Env architecture with an open conformation. The three masses are only connected in the lower part. The bottom row depicts the superposition of the density maps. Note the high overall congruency; the subtle differences in the tilting of the upper masses and the rotation of the triangular lower mass are not significant at the present resolution of 25 Å.