Prokineticin 2 is a novel 81 amino acid peptide with a complex folding structure including five disulphide linkages. It is a mammalian homolog to the mamba snake venom protein mamba intestinal toxin 1 and the frog protein Bv8, and has been demonstrated to be a paracrine mediator of gastrointestinal smooth muscle contractility.PK2 is expressed throughout the mammalian organism; however, in the brain, its expression is specifically concentrated in the suprachiasmatic nucleus (SCN). PK2 is rhythmically expressed in the SCN, transcriptionally regulated by clock control genes, and has the capacity to strongly suppress nocturnal locomotor activity. PK2 is one of the first neuropeptides that appears to directly convey circadian rhythm signaling to other CNS structures and, therefore, it may be a critical output molecule regulating biological rhythms.