The oxidation of protein-bound methionine to form methionine sulfoxide has traditionally been regarded as an oxidative damage. However, growing evidences support the view of this reversible reaction also as a regulatory post-translational modification. Thus, the oxidation of methionine residues has been reported to have multiple and varied implications for protein function. However, despite the importance of this modification and the abundance of reports, all these data are scattered in the literature. No database/resource on methionine sulfoxidation exists currently. Since this information is useful to gain further insights into the redox regulation of cellular proteins, we have created a primary database of experimentally confirmed sulfoxidation sites.