TXN, a small molecule selenium-containing protein with a molecular weight of approximately 12 kDa, forms the thioredoxin system together with nicotinamide adenine dinucleotide phosphate-oxidase (NADPH) and thioredoxin reductase (TRXR), which is one of the two redox regulatory systems supporting tumor growth. This evidence concerns the gene PRDX5 and neoplasm.