Structural Classification of Protein - Stable Domain Identifier

A 7-character sid consists of "d" followed by the 4-character PDB ID of the file of origin, the PDB chain ID ('_' if none, '.' if multiple as is the case in genetic domains), and a single character (usually an integer) if needed to specify the domain uniquely ('_' if not). Sids are currently all lower case, even when the chain letter is upper case. Example sids include d4akea1, d9hvpa_, and d1cph.1.