a database of known and predicted protein-protein interactions. the interactions include direct (physical) and indirect (functional) associations; they are derived from four sources: genomic context, high-throughput experiments, (conserved) coexpression, and previous knowledge. the database currently contains 1,513,782 proteins in 373 species. especially, the database provides interactions for synechocystis sp. pcc 6803. ctfbase: ctfbase contains 1288 putative transcription factors (tfs) identified from 21 fully sequenced cyanobacterial genomes. through its user-friendly interactive interface, users can employ various criteria to retrieve all tf sequences and their detailed annotation information, including sequence features, domain architecture and sequence similarity against the linked databases. furthermore, ctfbase also provides phylogenetic trees of individual tf family, multiple sequence alignments of the dna-binding domain and ortholog identification from any selected genomes.