Thioredoxin reductase (TrxR) is the first selenoenzyme containing selenocysteine in the active center and FAD as a second prosthetic group. TrxR catalyses the NADPH-dependent reduction of tioredoxin and of many other physiologically important substrates. TrxR exhibits many-fold increased activity in tumor cells and stimulates their proliferation as well the phenotype changes. Some gold compounds and a number of other clinically and experimentally used drugs have been shown to inhibit TrxR. The involvement of TrxR/Trx/NADPH system in broad spectrum of cellular processes renders it potential target for therapeutic approaches.