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Hi all, This is how I understand tNCS ... please correct me if I get this wrong. :o) I think tNCS is indeed NCS, the problem is that molecules related by tNCS are almost in the right position to be considered crystallographic symmetry molecules, except that they are usually off by a small translation (I think a small rotation could also provide this?) and therefore they are not crystallographic symmetry mates. When the difference is small enough it leads to misindexing and the resulting AU is approximately half as big as it should be in one direction. The resulting map isn't very good as it is the average of superimposing the two halves of the real AU. The resulting R factors are also quite high since the two halves do not superimpose perfectly because of the translation. If this is correct, Sudipta, then you might still have high R factors because your cell parameters are wrong (e.g. one of the cell edges should be twice as big) ... or maybe because the symmetry of your chosen space group is too high. To check the space group I would do as Herman suggested. Integrate the data again but in P1 (don't just expand it from P21212 to P1), find all the molecules via MR in the P1 data set and then do some rounds of refinement without using the NCS refinement option. You can then feed the resulting structure into ZANUDA, which will reduce it from P1 to all the possible higher symmetry space groups, compare the results and then suggest the best space group. ZANUDA hasn't always worked that well for me, but it has helped me in more than one occasion. When you index the data again to integrate it in P1, look at the table of possible solutions and see if there is one with a cell edge that's twice the size of your current ones. It might be worthwhile checking that solution even if the programme initially gives you a higher penalty than for your current solution. I hope this helps, Tony ------------------------------------------------------ Dr. Antonio Ariza University of Oxford Sir William Dunn School of Pathology South Parks Road Oxford OX1 3RE ________________________________