We cloned human thioredoxin as adult T cell leukaemia (ATL)-derived factor (ADF), an inducer of IL-2R/alpha produced by human T cell leukaemia virus type-I (HTLV-I)-transformed cells [18,21]. Thioredoxin (Trx) was originally reported as a hydrogen donor to ribonucleotide reductase, an essential enzyme for DNA sysnthesis in Escherichia coli. Trx has an active site with two cysteine residues; -Cys-Gly-Pro-Cyswhich are well conserved from eukaryotes to prokaryotes. Reduced Trx has dithiols and oxidized Trx has a disulfide bond in this active site. Oxidized Trx is reduced by NADPH and thioredoxin reductase [5]. Recent studies have shown that reactive oxygen species (ROS) generated by a variety of oxidative stresses are not only harmful to the cells but also important in signal transductions of cellular activation and cell death. Therefore, we have proposed that the redox (reduction/oxidation) status balanced by generated ROS and endogenous antioxidants plays a crucial role in the regulation of signal transduction in biological responses. The Trx system composed by Trx reductase, Trx and peroxiredoxin is imoportant in regulating the redox balance (Fig. 1). There is growing evidence that redox regulation by the Trx system as well as the GSH system plays crucial roles in biological responses against oxidative stresses.