"SCOP contains the domains of all PDB entries available at the time of the current release's construction. For each of these entries a coordinate file is available and can be displayed via the various graphical interfaces. The sequence of each protein chain has also been extracted. The release also contains many literature references. These are structures that have been published in sufficient detail to be classified in SCOP, but where the coordinates are not yet available from PDB. Whereas PDB entries are identified by a 4 letter code starting with a digit, these literature entries have been given 4 letter codes starting with the letter s followed by 3 digits, e.g. s149. This code is arbitrary and may change between releases. Entries included in SCOP in this way tend only to be those structures that are significantly different from any structure already in PDB. In the absence of a PDB header, a pseudo header file has been constructed. These header files contain the reference; the SWISSPROT identifier for the sequence; the fragment of the sequence for which structural information exists (where known) and the domain classification of each part of this sequence fragment in SCOP. (Note that occassionally some of this information is incomplete where there was insufficient information about the sequence in the orginal reference.)"