The MOLECULE_KEY is a DL_FIELD jargon, which is the unique code corresponds to a MOLECULE_TYPE (or the name of a MOLECULE). MOLECULE_KEY is equivalent to the amino acide residue code, that appears in PDB protein files. For example, ALA refers to alanine residue and this is also the alanine amino acid residue MOLECULE_KEY within the DL_FIELD context.

Of course, it does not confined only to proteins. All MOLECULE_TYPE must correspond to a unique MOLECULE_KEY. Although DL_FIELD allows longer string of code, it is a common practice to confine the length up to four characters because this is the maximum permissible length allow in the PDB format.

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