Detecting and overcoming crystal twinning

Twinning is fairly common in protein crystals. In its merohedral form, twinning is not apparent in the diffraction pattern, but the observed intensities do not represent individual crystallographic intensities. Since partial twinning (twin fraction less than 1/4 ) and perfect twinning (twin fraction of 1/4 ) can both be identified relatively easily by examining intensity statistics, the appropriate tests should be performed routinely when working in space groups that support merohedral twinning. In the case of partial twinning, the true intensities can be recovered directly. If the twinning is perfect, it still is possible to determine and refine a structure without ever knowing the true crystallographic intensities individually. For molecular replacement this has been accomplished in practice, while for isomorphous replacement, the problem has been solved in theory but not yet in practice.