Thioredoxins (Trxs) are a group of small ubiquitous proteins that are key regulators of cellular redox balance. The mammalian Trx family includes the secreted and cellular protein (Trx-1), the mitochondria-specific Trx-2, the Trx-like cytosolic protein p32TrxL, a truncated version of Trx-1 known as Trx80, and the thioredoxin-related protein, TRP-14. Trx-1 is secreted by lymphocytes, hepatocytes, fibroblasts, and several tumor cells. Trx-1 acts as a growth factor an, antioxidant, a cofactor that provides reducing equivalents, and a transcriptional regulator. Expression of Trx-1 is increased under various stress conditions such as hypoxia and viral and bacterial infections. It also serves as a biomarker for rheumatoid arthritis.