The heart of MS proteomics DDA data continues to be the "search engine", a tool that interprets mass spectra to determine the peptide or peptides that yielded them. The most common type of search engine continues to be the sequence search engine, which uses a reference input list of protein sequences, usually in FASTA format, to define the search space of possible peptide sequences. Spectral library search engines and de novo search engines, which are less common, are also available with TPP and are described further below.