To facilitate identification of low-abundance cell surface proteins, in vitro trapping experiments were typically performed using Trx1 concentrations of 1-3 mM. However, when disulfide exchange was subsequently tested at lower concentrations, Trx1(CSAAA) concentrations in the low nanomolar range (4-40 nM) were found to give rise to the formation of proportional amounts of Trx1-CD30 mixed disulfide intermediates ( Figure 3B ), thus demonstrating that the observed interaction is compatible with the expected physiological concentration range of secretory Trx1 (see below).