In the course of the crystallization experiments, a single yellow crystal of wild-type AcmB was obtained in trigonal form with two molecules in the asymmetric unit (ASU), denoted as chains A and B in the PDB structure. Although a few hydrogen bonding interactions exist between the molecules, the size exclusion chromatography experiments and atomic force microscopy imaging showed that AcmB occurs as a monomer in solution (Figure ). The high similarity of monomers A and B was proved by the low value of r.m.s deviations for Cα atoms (0.174 Å). The subtle differences between chains A and B are correlated with the different number of bound ligands. Analysis of the crystal structure proved that a FAD molecule is present in both monomers, while an ADD ligand occupies the active site of molecule A. The electron density maps have very good quality for almost the entire protein molecules, except for short disordered regions at the N-terminus of both monomers (MSI of A and MSIE of B), as well as for side chains of a few residues (L194A, M180B, L194B and L198B). Those regions were excluded from the refinement. Data collection and refinement statistics are summarized in Table .