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NOTE: In the version of this Brief Communication initially published, an author name (Lukas Mueller) was incorrect. The correct author name is Lukas N Mueller. The error has been corrected in the HTML and PDF versions of the article.

We describe a method to identify cross-linked peptides from complex samples and large protein sequence databases by combining isotopically tagged cross-linkers, chromatographic enrichment, targeted proteomics and a new search engine called xQuest. This software reduces the search space by an upstream candidate-peptide search before the recombination step. We showed that xQuest can identify cross-linked peptides from a total Escherichia coli lysate with an unrestricted database search.