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Dear All, I have solved crystal structure of a protein at 1.8 Å resolution using molecular replacement. Interestingly I could see extra electron density at the C-terminus end. The extra electron density is like split going in to two directions.We have checked with MS on protein in solution for extra residues but we did not find any extra molecular weight. C-terminal sequence is verified on the DNA level by DNA sequencing. Protein took around 30 days to crystallize, it has PEG 3350 as precipitant, around 0.1 M of NaCl and 0.2 M bis-tris at pH 5.5. Does any body have idea about these crystallizing agents doing c-terminus modification? I am attaching a snap shot of the extra electron density. I would be happy to receive information about reference about main chain modifications in crystallization process. Kind regards & Wish you all a happy and successful new year Siva