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Model of the single-chain anti-fluorescein fragment. In the mutant His(H58)Ala, a His residue in the heavy chain is replaced by Ala. The C-terminal tail, consisting of an IgG3 upper hinge, a Gly-Ser spacer, a His6 tag, Gly4, and a single C-terminal Cys, is shown (for sequence see text).
