In our proposed mmCIF model, within the atom_site, we would encode compositional heterogeneity (bound or unbound or unknown for ligand 1) and conformational heterogeneity state1/2. For example, when the compositional column indicated a 'Bound', the corresponding conformational state 1 would indicate the alt A. However, when the ligand was not bound, the compositional state will be unbound but the conformational state 1 could indicate alt A or alt B. The occupancies of the conformations in the bound state would be restrained to sum to the bound occupancy. For residues that do not interact with the ligand, we would imagine that the compositional column would be blank or unknown (Supplementary File 2). This concept can be extended to subunits in assemblies from cryoEM data or covalent linkages, such as a post-translational modification, as presented in Figure .