Dear all, After over 15 years solving structures I have (knowingly) encountered my first twinned data set. Short history.. Processed synchrotron data with HKL2000 SG P 42 21 2 ( Cell 105.485, 105.485, 62.445, 90, 90, 90) Rmerge 6% to 2A Solved MR using Phaser ( 1 mol in ASU) and built using COOT Refined using Phenix to a 'final' Rwork 29.4% Rfree 31.7% The maps look good but not great. Spurious peaks in difference maps. Couldn't get R's down any further. Xtriage suggested 'Overmerging pseudo-symmetric or twinned data' Re-processed in P2 ( cell 105.299, 62.385, 105.548, 90, 90.021, 90) Two molecules in ASU Xtriage suggested the presence of 3 pseudo-merohedral twin operators (one 4-fold and two 2-folds) Refined with twin law h,-k,-l (one of the pseudo 2-folds) using Phenix Rwork 26% Rfree 28% (no further building carried out at the moment) Wondered if it was a higher space group so.... re-processed in P222 (again Rmerge 6% to 2A) and did MR with Phaser in all the alternative space groups. The highest LL-gain was for P 21 21 21 (cell 62.377 105.297 105.546 90 90 90) I re-processed the data in P 21 21 21, all the systematic absences looked 'absent'. Xtriage suggested the presence of only one 4-fold Pseudo-merohedral twin law h,l,-k, R obs 5.1% Simulated annealing and ADP refinement in Phenix with twin law h,l,-k gave Rwork 33% Rfree 40%. Here are my questions... 1) Should I go for P2 due to the low R-factors and build from my good model ( beta angle of 90.021 seems unlikely ) 2) Should I try other possible SG ( HKL2000 suggested, primitive tetragonal, primitive orthorhombic, C centred orthorhombic, and monoclinic). 3) I'm concerned that my free-R reflections will be screwed-up since I have pseudo-merohedral twinning and some Rwork reflections may be the same as Rfree ones. I have read that Phenix is rather clever and chooses the highest observed symmetry to ovoid this problem but I can't see it being reported in the log-file that, in the case of P2, pseudo P42 21 2 is used for Rfree calculations. All help is much appreciated Mike -- Michael Gorman PhD St. Vincent's Institute of Medical Research 9 Princes Street Fitzroy Victoria 3065 Australia Tel : (03) 9288 2519