Selenocystine, also known as 3,3'-diselenodialanine, belongs to the class of organic compounds known as alpha-amino acids. These are amino acids in which the amino group is attached to the carbon atom immediately adjacent to the carboxyl group (alpha carbon).  More specifically, selenocystine is a diselenide consisting of two selenoamino acids that are attached together at their selenium atoms.  This particular selenoamino acid is selenocysteine, the selenium analogue to cysteine (selenium being the element directly beneath sulphur in the periodic table); likewise, selenocystine is the selenium analogue to cystine.  Since each constituent amino acid has a stereocentre, there are three different stereoisomers of selenocystine: D-selenocystine, L-selenocystine, and meso-selenocystine, the first two of which are optically active.  Like other amino acids, L-selenocystine is the most common form within organisms; however, the D- and meso- forms have also been found (PMID: 30920149).  Selenocystine is a solid that is moderately soluble in water.  Due to the reactivity of selenocysteine, it is rarely encountered; rather, cells store selenium in the less reactive oxidized form of selenocystine or in a methylated form, such as selenomethionine (DOI: 10.1007/978-3-319-92405-2_3).  When cells are grown in the absence of selenium, translation of selenoproteins terminates at the UGA codon, resulting in a truncated, non-functional enzyme.  Unlike other amino acids present in biological proteins, selenocysteine is not coded for directly in the genetic code. Rather, the tRNA-bound seryl residue is converted to a selenocysteine residue by the pyridoxal phosphate-containing enzyme selenocysteine synthase (PMID: 17194211).  Kurt Franke et al. indicated that there was evidence that selenium was in a form similar to that of cysteine, predating Thressa Stadtman’s discovery of the 21st amino acid by four decades (PMID: 26949981; J. Biol. Chem. 111:643).  Selenocysteine may be denoted by the short forms Sec, U, or SeCys (Cys is used for cysteine), whereas selenocystine may be denoted by SeCys2.  However, the literature sometimes uses SeCys for selenocystine and may cause confusion.  Selenocystine has been found in animals, plants, and bacteria.  It is being researched as treatment for cancer and for its antioxidant properties (PMID: 24763048, 24030774).  Selenium, in its various forms such as selenocystine, is essential for many species, including humans, yet it is also toxic to all organisms; hence, it has come to be referred to as the “essential poison” (PMID: 26949981; 6679541).