MODOMICS is an extensive database that provides comprehensive information on RNA modifications, including chemical structures of modified ribonucleosides, their biosynthetic pathways, RNA-modifying enzymes, and the location of modified residues in RNA sequences. The 2013 update includes new features such as a census of human and yeast snoRNAs involved in RNA-guided RNA modification, a section on the 5′-end capping process, and a catalogue of 'building blocks' for the chemical synthesis of modified nucleosides. The database has been updated with newly identified modified ribonucleosides and over 100 functionally and structurally characterized proteins from various organisms. It also includes experimentally mapped modified nucleosides in snRNAs and snoRNAs, and expanded rRNA and tRNA sequences. MODOMICS offers enhanced search capabilities, including BLAST and PARALIGN searches, and improved user interface features. The database aims to facilitate research on RNA modification pathways and related biological processes.MODOMICS is an extensive database that provides comprehensive information on RNA modifications, including chemical structures of modified ribonucleosides, their biosynthetic pathways, RNA-modifying enzymes, and the location of modified residues in RNA sequences. The 2013 update includes new features such as a census of human and yeast snoRNAs involved in RNA-guided RNA modification, a section on the 5′-end capping process, and a catalogue of 'building blocks' for the chemical synthesis of modified nucleosides. The database has been updated with newly identified modified ribonucleosides and over 100 functionally and structurally characterized proteins from various organisms. It also includes experimentally mapped modified nucleosides in snRNAs and snoRNAs, and expanded rRNA and tRNA sequences. MODOMICS offers enhanced search capabilities, including BLAST and PARALIGN searches, and improved user interface features. The database aims to facilitate research on RNA modification pathways and related biological processes.