PDBChainSaw database schema. The database schema of the tables in PDBChainSaw models the one-to-many relation between structures and their chains. Entities in the table "chsaw_pdb" are uniquely identified with the four-letter PDB code. Entities in the table "chsaw_chain" are linked to their parent structure through the PDB code and add their single character chain identifier to their unique identifier field named "chainid". Since chain identifiers may appear to be of lower case or non-alphanumeric, another identifier "chainid2" was set to the numerical value of the chain identifier to allow for unique formatting in FASTA files.