Top-down protein identification. Top-down protein identification.A, charge deconvoluted and deisotoped tandem mass spectrum of intact and acetylated thymosin beta-4. (only b and y ions are shown, for a completely annotated spectrum see supplemental Fig. S3). B, comparison of top-down protein identifications made by searching the human IPI database (87,061 entires) or subsetDB (5455 entires) showing that almost all proteins can be identified in the 15x smaller subsetDB. C, distribution of precursor ion masses leading to a successful top-down identification. D, normalized sequence coverage plot of top-down identified proteins in which each line represents one protein. The darker the gray scale density, the more spectra matched to the respective region of the protein indicating that most proteins identified from MALDI IMS samples are fragments of the original protein sequence. Stefan K. Maier et al. Mol Cell Proteomics 2013;12:2901-2910 © 2013 by The American Society for Biochemistry and Molecular Biology, Inc.