The PDB has a field on CRYST1 records that is called Z but it is not the same as the crystallographic Z. Here is the definition from the PDB format description: The Z-value is the number of polymeric chains in a unit cell. In the case of heteropolymers, Z is the number of occurrences of the most populous chain. I do not think there is a need to have a discussion about the meaning or the usefulness of this parameter; it has been present in PDB entries as long as the present format has existed. What would be appreciated would be the addition of a field in the mmCIF dictionary that would hold this quantity and would facilitate the translation of PDB to mmCIF and vice versa. Frances C. Bernstein