For their contributions to the development of the mmCIF dictionary we greatfully acknowledge the members of the Macromolecular CIF working group: Paula Fitzgerald at Merck Research Laboratories, Phil Bourne at San Deigo Supercomputer Center, and Helen Berman at Rutgers. Thanks also go to Brian McMahon at the IUCr Office in Chester for many valuable discussions on core dictionary compatibility. This work was supported by the NSF (BIR9305135 & BIR9510703).