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With the development of methods to crystallize proteins to sizes suitable for X-ray analysis, structures of many proteins have been determined to elucidate the relationships between their structures and functions. However, crystals of biological macromolecules have been frequently found to be twinned by adherence of two crystal lattices. Some crystals show splitting of diffraction spots owing to the different tilts of the two lattices. Others pretend to be single crystals with no split spots, and their symmetries of intensity distribution vary with every data set. These latter have been called hemihedral, and in them the unique axes of the two crystals are exactly reversely parallel with each other. The observed intensities were found to be detwinned by the least-squares method after Britton's algorithm [Britton (1972). Acta Cryst. A28, 296-297], when several sets of intensity data were observed. Fortran programs have been successfully used to determine the scale factors and degrees of twinning from the data of hydroxylamine oxidoreductase. The analysis of data collected at various positions of one crystal of this enzyme suggested a heterogeneous distribution of twinning in the crystal.
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