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X-ray crystallographic model of influenza virus B/Beijing/87 NA (Protein Data Bank reference 1NSC). The amino acids at positions 203 and 375, which have coevolved, as well as residues D151 and T225 (positions 201, 377, 148, and 223, respectively, in the numbering for influenza B virus), were mapped. Since positions 203 and 375 fall on opposite sides of the head, they are clearly not compensatory variations.
