Frances has raised the issue of the PDB Z value and a place to put it in mmCIF. Personally I do not see a lot of use for the quantity -- it doesn't give the crystallographic multiplicity of asymmetric units, and for heteropolymers seems to me to be actively misleading. _cell.formulat_units_Z is rooted in small molecule crystallography, as is essentially all of the chemical_formula material. It seems to me that the ENTITY data items were defined because macromolecular crystalls are not as well defined chemically as small molecule crystals, and if we need a placeholder for the PDB Z value we should put one in. I don't think we should revise the definitions in a way that will make mmCIF less compatible with CIF. Would revision of the definition of _cell.formula_units_Z do this? Lynn Ten Eyck For context, Frances Bernstein wrote >_cell.formula_units_Z is defined as: > >; The number of the formula units in the unit cell as specified > by _chemical_formula.structural, _chemical_formula.moiety or > _chemical_formula.sum. >; > >But when I look at _chemical_formula I find: > >; Data items in the CHEMICAL_FORMULA category would not, in > general, be used in a macromolecular CIF. See instead the > ENTITY data items. > >which seems to imply that one should not be using _cell.formula_units_Z >in mmCIF. > >Herbert pointed out that if one were to revise the definition of >_cell.formula_units_Z, it could be used to hold the PDB Z value. >Apparently Phil Bourne assumes that _cell.formula_units_Z is meant to >hold the PDB Z value and he uses it for that purpose in pdb2cif.