AH-DB (Apo and Holo structures DataBase) contains 562,424 apo-holo pairs of 3,369 proteins from 677 organisms. These pairs can be categorized into two groups. The first group considers the ligand and ion mapping, namely the joint complex must have all the ligands and ions of the corresponding core complex. If the joint complex has more ligands and ions than the corresponding core complex, the extra ligands and ions would be regarded as added molecules. The second group, on the contrary, does not consider the ligand and ion mapping, namely the paired core and joint complexes may have different ligands and ions. No added ligand and ion in such apo-holo pairs. The charts below provide a detailed breakdown of the apo-holo pairs in AH-DB based on their species, complex size and added molecule types.