Twinning is fairly common in protein crystals. In its merohedral from, 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 1/2) and perfect twinning (twin fraction of 1/2) 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.
