Bombesin (BN), a neuropeptide that originally was purified from the skin of the European frog (Bombina bombina). It has many biological effects including hormone release, stimulation of pancreatic enzyme secretion, gallbladder contraction and bronchoconstriction. Bombesin is a highly specific marker of neuroendocrine differentiation and thus a valuable tumor marker. It activates three different G-protein-coupled receptors known as BBR1, -2, and -3.