We have recently crystallized a recombinant protein produce from E. coli, and determined the structure at 1.9 Å. The asymmetric unit contains two protein monomers. Beyond our expectation, strong Fo - Fc density is present at a cleft of one subunit, but not in the other. Density maps are given in the snapshots attached to this email. We tried to model Tris or Bis-tris propane that were used as the purification and crystallization buffers, but apparently either of them poorly fitted in this density. The molecule that can be modeled here seems more likely to be a ligand comprising 3 or 4 rings with good planarity, however we did not add any additives in our crystallization trials. So we think it should be something from E. coli, which has high affinity to our protein. I wonder if anybody can figure out which molecule well fits to the electron density.