To demonstrate by an independent method that Trx1 attacks and breaks a disulfide bond in CD30, we used thiol-specific cell surface biotinylation to verify that Trx1 activity generates free thiols within the CD30 ectodomain (Supplementary Figure S3 ). Subsequent analysis of CD30 cell surface expression by flow cytometry and fluorescence microscopy revealed that a brief treatment of CD30 þ cells with wild-type Trx1 led to the complete loss of CD30 recognition by the Ki-1 antibody ( Figure 5A , upper panel and Figure 5B , second column). Similar results were obtained when another monoclonal antibody against CD30, MAB229 (R&D Systems; Clone 81337), was used to examine CD30 expression ( Figure 5A , middle panel). Under the same conditions, recognition by the Ber-H2 antibody was only slightly affected ( Figure 5A , lower panel and Figure 5B , third column), indicating that Trx1-mediated disulfide bond reduction (and possibly rearrangement) induces a structural alteration in the CD30 ectodomain, which disrupts or conceals the Ki-1 epitope.