The simplest example is an apo protein with alternative conformations of single residues or sections of residues. Currently, these could be captured by alternative conformations or increased B-factors. In our proposed mmCIF format, the first conformational data category would represent the alternative positions of an individual residue (Figure ) or multiple residues, such as a loop (Figure ). We envision this 'conformational state 1' category to be within the atom_site loop of the mmCIF dictionary and as optional. We would allow users to use any REGEX categories to identify each conformation and have an additional data category with the description for each ID to inform others of what each state means. In this scenario, refinement software would work exactly as it is now by constraining each atom's occupancy to sum to one, but the altloc names will be longer than a single character. We also propose that 'altloc' in the current mmCIF format be moved into the conformational state 1 data category. Further, existing ensemble structures could be trivially converted to this data format by having each model in the ensemble have a different identifier in the conformational state 1 data category.