Echoing whats been said: 1- Are you sure your crystal really is in C 2 2 21? If so How good is your data (completeness, Rmerge, etc...) 2-Could have twinning? I recently just got done working on a structure that could be scaled in C 2 2 21 but turned out to really be an almost perfect P21 twin. (of course in monoclinic there are certain conditions for twinning...) 3- How good is your model, is it complete, all the waters? Missing protein or DNA? HTH
Yuri
