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The crystal structure of gpD, the capsid-stabilizing protein of bacteriophage λ, was solved by multiwavelength anomalous diffraction (MAD) for a selenomethionine (SeMet) derivative of the protein at 1.8 Å resolution, using crystals in space group P21 [Yang et al. (2000), Nature Struct. Biol. 7, 230–237]. Subsequent analysis showed that the crystals of both the original protein and the SeMet derivative were pseudo-merohedrally twinned with a twinning fraction ≃ 0.36, owing to the near-identity of the a and c axes. An analysis of the crystal structure solution is presented and the utility of twinned crystals for solving the structure using MAD and of different phasing strategies is discussed; the results obtained with several software packages are compared.
Keywords: twinning; MAD; gpD.

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