Despite significant advances in the identification of known proteins, the analysis of unknown proteins by MS/MS still remains a challenging open problem. Although Klaus Biemann recognized the potential of MS/MS for sequencing of unknown proteins in the 1980s, low throughput Edman degradation followed by cloning still remains the main method to sequence unknown proteins. The automated interpretation of MS/MS spectra has been limited by a focus on individual spectra and has not capitalized on the information contained in spectra of overlapping peptides. Indeed the powerful shotgun DNA sequencing strategies have not been extended to automated protein sequencing. We demonstrate, for the first time, the feasibility of automated shotgun protein sequencing of protein mixtures by utilizing MS/MS spectra of overlapping and possibly modified peptides generated via multiple proteases of different specificities. We validate this approach by generating highly accurate de novo reconstructions of multiple regions of various proteins in western diamondback rattlesnake venom. We further argue that shotgun protein sequencing has the potential to overcome the limitations of current protein sequencing approaches and thus catalyze the otherwise impractical applications of proteomics methodologies in studies of unknown proteins. Molecular & Cellular Proteomics 6:1123-1134, 2007.Current approaches to proteomics focus on the reliable identification of proteins under the assumption that all proteins of interest are known and present in a database. However, the limited availability of sequenced genomes and multiple mechanisms of protein variation often refute this assumption. Well known mechanisms of protein diversity include variable recombination and somatic hypermutation of immunoglobulin genes (1). The vital importance of some of these novel proteins is directly reflected in the success of monoclonal antibody drugs such as Rituxan TM , Herceptin TM , and Avastin TM (2, 3), all derived from proteins that are not directly inscribed in any genome. Similarly multiple commercial drugs have been developed from proteins obtained from species whose genomes are not known. In particular, peptides and proteins isolated from venom have provided essential clues for drug design (4, 5); examples include drugs for controlling blood coagulation (6 -8) and drugs for breast (9, 10) and ovarian (11) cancer treatment. Even so, the genomes of the venomous snakes, scorpions, and snails are unlikely to become available anytime soon.Despite this vital importance of novel proteins, the mainstream method for protein sequencing is still initiated by restrictive and low throughput Edman degradation (12, 13), a task made difficult by protein purification procedures, posttranslational modifications, and blocked protein N termini. These problems gain additional relevance when one considers the unusually high level of variability and post-translational modifications in venom proteins (14,15). Moreover the common labor-intensive approach of DNA cloning and sequencing...