Twinning is a crystal-growth disorder in which the specimen is composed of distinct domains whose orientations differ but are related in a particular, well-defined way. Twinning, which is a known problem in protein crystallography, usually hampers high-quality crystal structure determination unless it is detected and either avoided or corrected. Although effective computational methods have been developed for the determination of structures using data from twinned crystals (known as ‘detwinning’), if possible it is preferable to obtain crystals that are not twinned.In some cases, optimising the length of the protein fragment used for crystallisation can lead to the growth of untwinned crystals, but this is a time-consuming process.