MODOMICS is a comprehensive database of RNA modifications, providing information on the chemical structures of modified ribonucleosides, their biosynthetic pathways, RNA-modifying enzymes, and the locations 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 new section on the 5'-end capping process, and a catalog of 'building blocks' for the chemical synthesis of modified nucleosides. The database has been updated with new modified ribonucleosides and over 100 functionally and structurally characterized proteins. RNA sequences have been expanded, including snRNAs and snoRNAs with experimentally mapped modified nucleosides. Each record is cross-referenced to other databases and key publications. New search options, including BLAST and PARALIGN, have been implemented to facilitate data access. The database also includes information on RNA sequences, proteins involved in RNA modification, and snoRNAs. The MODOMICS database is freely accessible and provides detailed information on RNA modifications, including chemical structures, sequences, and enzymatic pathways. The database is continuously updated with new modifications and enzymes, and future plans include expanding the database to include more RNA sequences and integrating it with other RNA metabolism databases. The database is a valuable resource for researchers studying RNA modifications and their biological functions.MODOMICS is a comprehensive database of RNA modifications, providing information on the chemical structures of modified ribonucleosides, their biosynthetic pathways, RNA-modifying enzymes, and the locations 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 new section on the 5'-end capping process, and a catalog of 'building blocks' for the chemical synthesis of modified nucleosides. The database has been updated with new modified ribonucleosides and over 100 functionally and structurally characterized proteins. RNA sequences have been expanded, including snRNAs and snoRNAs with experimentally mapped modified nucleosides. Each record is cross-referenced to other databases and key publications. New search options, including BLAST and PARALIGN, have been implemented to facilitate data access. The database also includes information on RNA sequences, proteins involved in RNA modification, and snoRNAs. The MODOMICS database is freely accessible and provides detailed information on RNA modifications, including chemical structures, sequences, and enzymatic pathways. The database is continuously updated with new modifications and enzymes, and future plans include expanding the database to include more RNA sequences and integrating it with other RNA metabolism databases. The database is a valuable resource for researchers studying RNA modifications and their biological functions.