Our proposed amendments to the PDBx/mmCIF model are designed to better represent conformational and compositional heterogeneity by capturing these heterogeneities with new data categories in a PDBx/mmCIF (Supplementary File 2). This modification will separate out conformational and compositional heterogeneity and allow for information to be transferred about the interconnectedness of this heterogeneity. We propose adding these data categories onto the atom_site loop within the PDB, connecting each atom in a structure to its conformational or compositional state. While a data category in the PDBx/mmCIF dictionary allows for more description of conformational heterogeneity (alt_ens), this does not separate conformational versus compositional heterogeneity, nor does it allow for layering of conformational heterogeneity. This improved data structure will accelerate the development of new tools and create representative training datasets for structural ensemble prediction. Alongside this format, infrastructure changes to refinement, visualization, and validation tools are likely needed. We have outlined changes here to be maximally backward compatible for refinement and visualization software; for example, the hierarchical representations can be implemented, at first, as parsable group occupancy definitions. This new format should also help interconvert existing multiconformer and ensemble-based models. Such interconversion enables manual manipulations, such as in Coot 10 , or data mining approaches. We envision refining mmCIF further to more effectively correlate grouped data, including timeresolved techniques, ligand soaking experiments, or EM classification/reconstructions . For example, a 'perturbation' data category could connect to specific structure factors or real-space maps, enabling restrained refinements of coordinates.