              Data items in the STRUCT_BIOL category record details about\
               the structural elements that form each structure of biological\
               significance.\
\
               A given crystal structure may contain many different biological\
               structures. A given structural component in the asymmetric\
               unit may be part of more than one biological unit. A given\
               biological structure may involve crystallographic symmetry.\
\
               For instance, in a structure of a lysozyme-FAB structure, the\
               light and heavy chain components of the FAB could be one\
               biological unit, while the two chains of the FAB and the lysozyme\
               could constitute a second biological unit.