Thioredoxin (Trx) is a small multifunctional protein with a redox active dithiol/disulfide in the active-site sequence Cys-Gly-Pro-Cys. Trx functions as a key molecule in the maintenance of cellular redox balance. In addition to the cytoprotective action against oxidative stresses, Trx is involved in various cellular processes including gene expression, signal transduction, proliferation and apoptosis. Recently, various proteins sharing Trx-like active-site sequences have been found and classified as part of the Trx superfamily. Trx and these Trx-related molecules constitute a cellular redox regulation system, and further studies to clarify their biological functions will provide new therapeutic strategies for the disorders caused or complicated by oxidative stresses. (c) 2002 Prous Science. All rights reserved.