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Dear all, I am trying to solve a structure at 2.05 A resolution by molecular replacement. The space group seems to be P21, with unit cell dimension 52.63, 29.43, 104.970 and beta = 95.60. Only one copy of the protein should be present in the asymmetric unit, with 58% of solvent content. The search model used for MR is a truncated construct of the same protein, comprising more that 60% of the residues. However, no convincing MR solution is found (I used phaser, molrep, epmr and also mr.bump). No solutions refine to R and Rfree lower than 51-52%. The CCP4 documentation about twinning states that "Monoclinic with na + nc ~ a or na + nc ~ c can be twinned". This is not clear to me, but I have c = 2a, and therefore n = 2/3. Nevertheless all the tests run by ctruncate (and sfcheck) exclude twinning. The observed cumulative distribution for |L| almost overlap the expected untwinned, and the observed cumulative intensity distribution is not sigmoidal at all (actually it is growing faster that the theoretical). Also the acentric and centric moments exclude twinning, for example the acentric: <E> = 0.858 (Expected value = 0.886, Perfect Twin = 0.94) <E**3> = 1.442 (Expected value = 1.329, Perfect Twin = 1.175) <E**4> = 2.438 (Expected value = 2, Perfect Twin = 1.5) Both ctruncate and sfcheck found a pseudo-translation vector: ctruncate (0.050, 0.000, 0.957), ratio 0.23 sfcheck (0.954, 0.000, 0.040), ratio 0.218 However a second copy cannot be present in the asymmetric unit (there would be 16% of solvent content). Since the protein is expected to form a coiled-coil, I think that the detected pseudo-translation arises from the helices. Alternatively, it is possible that the space group is wrong? And if so, how can I figure out the correct one? Thank you in advance, Michele