In practice, of course, such an exhaustive ground truth labeling of mass spectrometry data is impossible to obtain. However, at least three possible alternative evaluation strategies exist. The first solution is to use spectra that were generated from synthesized peptide sequences, such as those contained in the ProteomeTools database . This type of data is ideal because the generating peptide for each observed spectrum can be confidently identified. However, the data itself are somewhat unrealistic, as the spectra do not come from a complex mixture and hence exhibit lower noise levels than those typical for natural biological samples. Nonetheless, data from synthetic peptides have been used to train multiple de novo sequencing methods and have also been used for performance evaluation .