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This is an interesting case but your description lacked the diffraction part. Assuming no Heavy Atom sites in the native structure. Based on your description, another possibility may be Lattice Translocation. ----------------- Ref: J. Wang, S. Kamtekar, A. J. Berman and T. A. Steitz. Correction of X-ray intensities from single crystals containing lattice-translocation defects. Acta Cryst. (2005). D61, 67-74. ----------------- By the way, indexing with orthorhombic cell (137.1Å, 83.3Å, 169.8Å) should gain a large penalty since the new beta angle has a >5 degree tolerance from right angle, derived from your P21 cell constants (a, c and beta angle) (if they were well determined), implying this may not be the case. Good luck! Lijun Liu, PhD Institute of Molecular Biology HHMI & Department of Physics University of Oregon Eugene, OR 97403 541-346-5176 http://www.uoregon.edu/~liulj/ ---------------------- On Jan 6, 2009, at 8:35 AM, Stephen Hare wrote: Dear All, We are currently working on a structure of apparent P21 symmetry which has been solved by molecular replacement. The data are to 2.7Å but the Rfree will not drop below 30%. The density is clear for the model we have, however there is extra density that suggests a shift of the structure by 16Å in either direction - resulting in three possible overlapping positions for the structure. We assume this is the result of twinning. The unit cell dimensions are 102.7Å, 83.0Å, 115.3Å, 90°, 101.8°, 90°. Examining the data with phenix.xtriage also suggests pseudo translational symmetry with a separation of 16Å. A Patterson peak at 0.097, 0.000, -0.096 is approximately 30% of the origin peak, while a second peak of double the translation at 0.192, 0.000, 0.195 is 7% of the origin peak. The structure contains a dimer of dimers with an NCS 2 fold axis almost perpendicular to the crystallographic 2 fold. This NCS axis almost coincides with the diagonal between the A and C axes. A twin axis along the A C diagonal (l,-k,h) could explain the observed extra density, however this is not possible because A and C are different lengths. As a result of the NCS axis running almost perpendicular to the observed P21 axis, it is possible to merge the reflections in a larger orthorhombic unit cell - dimensions 137.1Å, 83.3Å, 169.8Å although here the Rmerge is higher and it is not possible to get a molecular replacement solution. Is it possible to define the (l,-k,h) twinning operator in our original unit cell? or have we missed the actual unit cell? Or....something else? Steve Stephen Hare PhD Post doctoral research associate Jefferiss Research Laboratories Wright-Fleming Institute Division of Medicine Imperial College London Norfolk Place London W2 1PG UK Phone: +44 (0) 20 7594 3908 Fax: +44 (0) 20 7594 3906 Lijun Liu, PhD Institute of Molecular Biology HHMI & Department of Physics University of Oregon Eugene, OR 97403 541-346-5176 http://www.uoregon.edu/~liulj/