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Hi Francis, The space group will not influence the Patterson, as both C222 and C2221 will become Pmmm in Patterson space. (Aside: You can change the packing pattern from one into the other by shifting the origin by x+0.5, I think, equivalent to adding 0.5 to the x-coordinate. However, you haven't got there yet!). The pattern you show is probably due to the dominance of one or a few reflections that is/are unusually large relative to the others. My guess, from the plot, is H=18, K=20. In a native Patterson, this would be highly unusual, but possible with translational NCS. If this map is some kind of difference Patterson, say anomalous, it is very easy to get such individual reflections that dominate (poor resolution, according to your hint). You have to be careful in selecting the refelctions with these low accuracy measurements. SCALEIT of the data set for analysis would give you a number of measures to select reliable data with: Highest index in each direction, highest and lowest I/sigma (or whatever the coefficient relates to), maximum and minimum numerical values, etc. If all these fail to weed out the erroneous data, you can still exclude specific reflections, provided you know which ones they are. Good Luck. Pierre Rizkallah ********************************************************************** Dr. Pierre Rizkallah, Senior Lecturer in Structural Biology, WHRI, School of Medicine, Academic Avenue, Heath Park, Cardiff CF14 4XN email: [log in to unmask] phone + 44 29 2074 2248 >>> Francis E Reyes <[log in to unmask]> 19/08/09 8:40 PM >>> Hi All Im receiving some strange patterns in my pattersons. Space group is C222 with no confidence (due to resolution) of systematic absences to transform to C2221. Thanks! FR