In polyadenylation, a poly-adenosine tail of about 200 adenylate residues is added by a nuclear polymerase post-transcriptionally. This is known as a Poly-A tail and is used for stability and guidance, so that the mRNA can exit the nucleus and find the ribosome. It is added at a polyadenylation site in the 3' untranslated region of the mRNA, cleaving the mRNA in the process. When there are multiple polyadenylation sites on the same mRNA molecule, alternative polyadenylation can occur. See polyadenylation for further details.