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
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