Detection of twinning in macromolecular crystallography

The merohedrally or pseudo-merohedrally twinned crystals cannot be identified during diffraction pattern inspection at the stage of data collection. Several methods for identifying twinning and estimating the twin fraction are suitable for macromolecular crystals, and all are based on the statistical properties of the measured diffraction intensities. They can be based on either the overall statistical properties of the measured reflection intensities or on the comparison of reflection intensities related by the twinning operation. The application of various tests for identification of twinning and estimation of twinning fraction is discussed, with examples of diffraction data from the Protein Data Bank. Twinning makes the solution of crystal structures more difficult, but once initially solved, the atomic models can be properly refined by the existing programs.