Cysteine has the same structure as serine, but with one of its oxygen atoms replaced by sulfur; replacing it with selenium gives selenocysteine. (Like other natural proteinogenic amino acids cysteine has (L) chirality in the older D/L notation based on homology to D and L glyceraldehyde. In the newer R/S system of designating chirality, based on the atomic numbers of atoms near the asymmetric carbon, cysteine (and selenocysteine) have R chirality, because of the presence of sulfur (resp. selenium) as a second neighbor to the asymmetric carbon. The remaining chiral amino acids, having lighter atoms in that position, have S chirality.)