Asymmetric Unit

From Proteopedia

The asymmetric unit (see definition in the next paragraph) is what is contained in the protein model that crystallographers publish in the Protein Data Bank. It is not to be confused with the Biological Unit, which may be the same, smaller, or larger than the asymmetric unit.

The asymmetric unit is the smallest portion of a crystal that, when duplicated and moved by crystal symmetry operations, can produce the unit cell of the crystal. The symmetry operations that are common for biological molecules are rotation, translation, and screw operations (which combine rotation and translation).

The unit cell is the smallest portion of a crystal that, when duplicated and translated, can generate the entire crystal.

Diagrams illustrating these concepts are available at RCSB Protein Data Bank.