Where does the data come from?

Each x-ray structure is solved in a particular crystal symmetry and the PDB requires authors to deposit only the smallest or asymmetric unit from any crystal form. As a result, the PDB entry in the archive often does not contain the assembly that is most likely to exist under physiological conditions.

In many cases, crystal symmetry operations need to be applied in order to obtain the biologically active form of a macromolecule or complex. Conversely, some macromolecules crystalise with several copies of the biologically relevant assembly in the asymmetric unit. This means that the deposited entry contains extra copies of the biological assembly. Contacts in the crystal hold these copies alongside each other but are too weak to survive in solution.