Urotensin-II (U-II) is a peptide ligand, initially isolated from the neurosecretory system of the Goby fish (Gillichthys mirabilis) Bern et al. 1969. For many years it was thought that U-II does not exhibit significant effects in mammalian systems; a view quickly overturned when it was demonstrated that Goby U-II produces slow relaxation of mouse annococygeus muscle, in addition to contraction of rat artery segments. In 1998, the cDNA encoding a U-II precursor was cloned in humans, unequivocally demonstrating its existence in mammalian species.As with other peptide ligands, U-II is synthesised from a larger precursor molecule, known as Prepro-urotensin-II, two isoforms have been identified in man of lengths 124 and 139 residues. Cleavage of either of these precursors produces identical, eleven residue, mature U-II peptides.