ProtCID contains comprehensive, PDB wide structural information on the interactions of proteins and individual protein domains with other molecules, including four types of interactions: chain interfaces, Pfam domain interfaces, Pfam peptide interfaces and Pfam ligand/nucleic acids interactions. A common interaction here indicates chain chain or Pfam domain domain interfaces that occur in different crystal forms or Pfam peptide or Pfam ligand interactions that occur in multiple homologous proteins.