The 7-character SCOP domain identifier consists of 'd', followed by the 4-character PDB ID of the file of origin, followed by the PDB chain ID ('_' if none, or '.' if multiple), followed by a single-digit integer to specify the domain in our database uniquely ('_' if no such domain exits). For example d1pjsb3, indicating the third domain in chain 'b' of the protein 1pjs.