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Hi Sudipta, You're correct in that you shouldn't use the twin refinement option to refine your structure if it isn't twinned, I simply mentioned using it as a quick way to test for twinning as the refmac log provides a very good estimate of twinning fractions if twinning is present. If it detects no twinning it will let you know as well ... but you obviously shouldn't use the output from that run if no twinning is observed ... even if the twin refinement lowers the R factors, as it will do this regardless of whether twinning is present or not. With tNCS a large cell is generally averaged into two smaller cells which are almost (but not quite) identical, which gives rise to poor maps and high R factors. However, since you seem to have detected the tNCS, I guess your indexing has found and measured the weak reflections that are generally missed in this case. Nonetheless, I would have a second look at your indexing and check that there aren't any weak spots between the intense ones that aren't being measured while you integrate. Other than that I guess there might be a problem with the choice of origen, which can happen when one of the cell edges is half the size of another one. Best, Tony ------------------------------------------------------ Dr. Antonio Ariza University of Oxford Sir William Dunn School of Pathology South Parks Road Oxford OX1 3RE ________________________________