To identify candidate Trx1 target proteins on the cell surface of lymphoid cells, we applied a trapping technique based on the reaction mechanism. This approach makes use of the finding that mutant thiol-dependent oxidoreductases lacking the C-terminal cysteine of the CXXC active site motif form long-lived mixed disulfide intermediates with target proteins. Thus, target proteins remain covalently linked to the mutant oxidoreductase and become amenable to isolation and analysis (principle shown in Figure 1A ). Kinetic trapping has been applied previously to identify interaction partners of Trx family members in plants (Motohashi et al, 2001) and in the secretory pathway of human lymphocytes . In these studies, the CXXC-based trapping technique identified both established and novel target proteins, subsequently confirmed by independent techniques, demonstrating the competence of this technique to identify bona fide interaction partners.