Figure 2: Automatic specificity expansion results in accurate localization of hydroxylation. ModLS localization with specificity expansion of known asparagine hydroxylation in the Asn-649 peptide of Rabankyrin (ANKFY1). This figure presents the various possible localizations considered by ModLS for hydroxylation on a single peptide to spectrum match. The original database search was conducted without specifying Asn hydroxylation as a variable modification and the search engine incorrectly assigned His oxidation (bottom row). Using specificity expansion ModLS correctly identifies the Asn-649 hydroxylation as the most likely, highest scoring, assignment (top row). This demonstrates how specificity expansion can identify and prevent reporting of PTM assignments to an incorrect amino acid.