The remaining neutralizing antibodies were HAI + and therefore bound to the HA globular head. Based on crosscompetition analyses, these antibodies fell into two groups binding nonoverlapping regions of the HA head, including epitope-2 and epitope-3 (Fig. 3 B and Fig. 4 A) . Indeed, using spontaneous escape mutant selection, we found that the EM4C04 mAb binds to the Sa region of the HA globular head (unpublished data). Thus, by proximity based on the competition assay (Fig. 4 A) , we can predict that all of the epitope-2 antibodies bind near the Sa/Sb region (including EM-4C04, 1009-3B06, and 1009-3F01).