Molecular model of Selenocysteine (C3. H7. NO2. Se), a proteinogenic but non-standard amino acid that exists in Archaea, Bacteria and Eukarya, but not in all organisms. It is the only amino acid containing an essential dietary micronutrient (selenium) as a constitutive component, the only amino acid encoded by a UGA codon (a stop codon), and the only amino acid synthesized on its tRNA in all domains of life. In humans it has been found in 25 selenoproteins and selenoenzymes important for fundamental cellular processes ranging from selenium homeostasis maintenance to the regulation of the overall metabolic rate. In all organisms that contain selenocysteine, both the synthesis of selenocysteine and its incorporation into a selenoprotein requires an elaborate synthetic and translational apparatus, which does not resemble the canonical enzymatic system employed for the 20 standard amino acids. Atoms are represented as spheres and are colour-coded: carbon