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.