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The HA in the pre- (A), intermediate (B), and post-fusion (C) states. The termini of HA1 (light blue) and HA2 are labeled as N1, C1, N2, and C2, respectively, in (A). The dotted lines in (C) indicate unresolved regions. The structures have all been aligned on the cyan helix, which is the only region in that domain that does not change during the transition.
