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Crystals of macromolecules often have two or more molecules per asymmetric unit, or contain domains of a macromolecule or a macromolecular complex that are structurally independent. In such cases the conventional molecular-replacement method attempts to determine the position of each structural unit independently. Typically, some parts of the structure can be determined more easily or more reliably than other parts. Methods are proposed whereby information from a part of a crystal structure that has been determined can be used to help determine the structure of the remainder. Two different strategies are discussed, `subtraction' and `addition'. With `subtraction' strategy the Patterson function of the known part of the structure is subtracted from the `observed' Patterson. This approach is found to be most effective in the context of the rotation function in that it eliminates peaks that are irrelevant to the desired solution. With `addition' strategy the structure factors of the known component are added to those of the search model. This procedure is most effective in the context of the translation function because it brings the structure factors calculated from the search model closer to those observed. Methods of applying the fast Fourier transform to facilitate these calculations are described. A number of examples are provided including structures of mutants of T4 lysozyme that might not have been solved without recourse to the proposed methods. A method of including information from a heavy-atom derivative in a translation function is also developed and shown to be superior in some situations to the conventional translation function.
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