L-Selenocystine is a diselenide-bridged amino acid that may be confused with selenocysteine (Sec), which is a rare amino acid featuring a single selenium atom. L-Selenocystine is a redox-active selenium compound that has both anti- and pro-oxidant actions. This compound can be reduced by low molecular thiols and disulfide reductases to Sec. It is reduced to Sec by mammalian thioredoxin reductase (apparent Km = 6.0 µM), and this property can be used to assay thioredoxin reductase activity. L-Selenocystine induces an unfolded protein response, ER stress, and large cytoplasmic vacuolization in HeLa cells and has cytostatic effects in a range of cancer cell types.