This polypeptide is subsequently cleaved into individual polypeptide chains. The ester bonds in polyethylene terephthalate may be cleaved by hydrolysis, or by transesterification. Once cyclization has taken place, the peptide is cleaved from resin by acidolysis and purified. These are then cleaved by an AP endonuclease. Monosaccharides can be linked together by glycosidic bonds, which can be cleaved by hydrolysis. In the Zincke disulfide cleavage reactions, disulfides are cleaved by halogens. This flap is then cleaved by endonucleases. Inside the RER, the signal peptide is cleaved to form proinsulin. IL-18 is also synthesized as a precursor which is cleaved by caspase-1. Ubiquitin is cleaved before terminal digestion by deubiquitinating enzymes. Optionally, disulfide bridges can be cleaved by reduction. Fumarate is then cleaved off forming adenosine monophosphate. This transcript is then cleaved by Cas proteins to form crRNAs. When the tags are not needed anymore, they can be cleaved off by a protease. There it is cleaved by ATP citrate lyase into acetyl-CoA and oxaloacetate.