MODOMICS is the first comprehensive database system for biology of RNA modification. It integrates information about the chemical structure of modified nucleosides, their localization in RNA sequences, pathways of their biosynthesis and enzymes that carry out the respective reactions (together with their protein cofactors). Also included are the protein sequences, the structure data (if available), selected references from scientific literature, and links to other databases allowing to obtain comprehensive information about individual modified residues and proteins involved in their biosynthesis.