YT cells respond to CD30 signals by downregulating the expression of cytotoxic effector molecules, including FasL, thus decreasing their cytotoxicity against Fas-expressing target cells (Bowen et al, 1993; Muta et al, 2000) . To test if Trx1 influences CD30-mediated suppression of cytotoxicity, we treated YT cells with Trx1(CCAAA) or Trx1(SSAAA) before stimulation with agonistic anti-CD30 antibody and quantified cytotoxicity against Cr-labeled Raji cells. Upon CD30 stimulation, cytotoxicity was markedly reduced ( Figure 7B , compare columns 1 and 4). The decrease in cytotoxicity was completely reversed by Trx1(CCAAA) but not the catalytically inactive mutant (SSAAA) ( Figure 7B , columns 5 and 6). As judged by RT-PCR, changes in cytotoxicity correlated with changes in FasL mRNA expression ( Figure 7B, lower panel) . These results confirm that the catalytic activity of Trx1 modulates CD30-dependent changes in cellular behavior and function.