The PSI Extended Fasta Format (PEFF) is a unified format for protein and nucleotide sequence databases to be used by sequence search engines and other associated tools (spectra library search tools, sequence alignment software, data repositories, etc). This format enables consistent extraction, display and processing of information such as protein/nucleotide sequence database entry identifier, description, taxonomy, etc. across software platforms. It also allows the representation of structural annotations such as post-translational modifications, mutations and other processing events. The proposed format has the form of a plain text file that extends the formalism of the individual sequence entries as presented in the FASTA format and that includes a header of meta data to describe relevant information about the database(s) from which the sequences have been obtained.