ppdb (http://www.ppdb.gene.nagoya-u.ac.jp) is a plant promoter database that provides promoter annotation of Arabidopsis and rice. The database contains information on promoter structures, transcription start sites (TSSs) that have been identified from full-length cDNA clones and also a vast amount of TSS tag data. In ppdb, the promoter structures are determined by sets of promoter elements identified by a position-sensitive extraction method called local distribution of short sequences (LDSS). By using this database, the core promoter structure, the presence of regulatory elements and the distribution of TSS clusters can be identified. Although no differentiation of promoter architecture among plant species has been reported, there is some divergence of utilized sequences for promoter elements. Therefore, ppdb is based on species-specific sets of promoter elements, rather than on general motifs for multiple species. Each regulatory sequence is hyperlinked to literary information, a PLACE entry served by a plant cis-element database, and a list of promoters containing the regulatory sequence.