The cysteine molecule plays an essential role in cells because it is part of proteins and because it functions as a reduced sulfur donor molecule. In addition, the cysteine molecule may also play a role in the redox signaling of different stress processes. Even though the synthesis of cysteine by the most abundant of the isoforms of O-acetylserine(thiol) lyase in the chloroplast, the mitochondria and the cytosol is relatively well-understood, the role of the other less common isoforms homologous to O-acetylserine(thiol)lyase is unknown. Several studies on two of these isoforms, one located in the cytosol and the other one in the chloroplast, have shown that while one isoform operates with a desulfhydrase activity and is essential to regulate the homeostasis of cysteine in the cytosol, the other, located in the chloroplast, synthesizes S-sulfocysteine. This metabolite appears to be essential for the redox regulation of the chloroplast under certain lighting conditions.