ABOUT 120,000 types of protein mole-cule have yielded up their structures to science. That sounds a lot, but it isn't. The techniques, such as x-ray crystallography and nuclear-magnetic resonance (nmr), which are used to elucidate such structures do not work on all proteins. Some types are hard to produce or purify in the volumes required. Others do not seem to crystallise at all-a prerequisite for probing them with x-rays. As a consequence, those structures that have been determined include representatives of less than a third of the 16,000 known protein families. Researchers can build reasonable computer models for around another third, because the structures of these resemble ones already known. For the remainder, however, there is nothing to go on.
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