The other two major deletions in the 3CSY structure (N278-R299 and A189-Y214) are situated at the midpoint of the structure just above the stalk (shown in pink in Figure 5 ). Inclusion of the KZ52 Fab in the docked structure demonstrates that this neutralizing epitope (from a human survivor) is localized on the side of the stem region of EBOV GP trimer at the base of the clubshaped head, and that the Fab domain lies close to the lipid envelope when bound, and approximately tangential to the viral envelope. The densities putatively corresponding to N278-R299 and A189-Y214 are close to the KZ52 neutralizing site, but do not obstruct antibody binding. The mucin-like domains are out of the way and cannot interfere stearically with KZ52 Fab binding. This is consistent with previous results indicating that KZ52 binding does not require cathepsin cleavage [48] . We have thus delineated the low resolution structure of the glycocalyx or ''glycan cap'' that covers the distal end of the uncleaved EBOV GP spike, which is consistent with a proposed role in immune evasion [42] . Our data will enable docking of future structures to investigate receptor binding, antigenicity, and fusion mechanisms.