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Crystals of the protein B-phycoerythrin from the red alga Porphyridium cruentum undergo twinning by merohedry in which the two members of the twin are related by rotation about (1,1,-1,0), a symmetry operation of the lattice but not of the crystals. Several methods are compared for estimation of the volume fractions of the two members of the twin so that measured data can be corrected for this twinning. The effect of these corrections on the final electron-density map is analyzed. Results show that when the volume fraction of the smaller crystal in a twinned specimen used for structure determination by multiple isomorphous replacement is 0-0.1, correction of diffraction data for twinning results in a small but significant improvement in the accuracy of the electron- density map.
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