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Few examples of pseudomerohedrally twinned macromolecular crystals have been described in the literature. This unusual phenomenon arises when a fortuitous unit-cell geometry makes it possible for twinning to occur in a space group that ordinarily does not allow twinning. Here, the crystallization, structure determination and refinement of the cocaine hydrolytic antibody 15A10 at 2.35 Å resolution are described. The crystal belongs to space group P21, with two molecules in the asymmetric unit and unit-cell parameters a = 37.5, b = 108.4, c = 111.3 Å and β fortuitously near 90°; the refined twinning fraction is α = 0.43. Interestingly, the non-crystallographic symmetry (NCS) and twin operators are nearly parallel, which appears to be a relatively frequent situation in protein crystals twinned by merohedry or pseudomerohedry.

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