Two consecutive stages of mass spectrometric fragmentation (MS3). The precursor of peptide DVPNSQPEMVEAVK (a; see insert) was selected for fragmentation from a full scan of mass to charge ratio range. The doubly charged y12 fragment ion (b) was subsequently fragmented. Characteristic pattern for charged directed fragmentation is observed in MS3 spectra (c) and confirms the identification of the above peptide. See Steen and Mann  for an introduction to peptide sequencing and confidence of peptide identification. MS, mass spectrometry.