Search for structures based on the number of entities.
Entities are the distinct chemical components of structures in the PDB. Unlike chains, entities do not include
duplicate copies. In other words, each entity in a structure is different from every other entity in the structure.
Hence, this search queries for structures based on the number of distinct chemical components they contain.
Chemical components can be of different molecular type. Polymer entities include Protein, DNA, and RNA.
Ligands are smaller chemical components that are not part of a polymer entity.
4HHB contains two protein entities: hemoglobin alpha chain and hemoglobin beta chain.
Choose Protein from the pull down menu and enter "Between 2 and 2". Press "Submit Query" to find all
structures that contain exactly 2 protein entities. The results include 4HHB.
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