Electron transfer dissociation (ETD) tandem mass spectrometry has been widely used to characterize primary structure of proteins,where matching c and z fragment ions are utilized to identify amino acid sequence and the type and location of post-translational modifications.Yet c and z ions are just about 30% of the total number of observed fragment ions.Uncover the identities of fragment ions other than c and z ions will greatly increase the confidence of identification without any extra effort on the experimental side.In this talk,I will present our efforts in manually interpreting every fragment ion in ETD spectrum of ubiquitin.Besides regular c and z ions,internal fragment ions,neutral loss of NH3,C02,CO,and their combinations were also observed.Some statistical abundance relationships among different ion types were found and confirmed in some other proteins.