Purity ~98% (HPLC). A potent antagonist highly selective for Mu opioid receptor (Ki = 0.96nM for u receptor and > 10,000nM for delta receptors). Often used for studying the functional roles of Mu opioid receptors in both central and peripheral nervous systems. Sequence: D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2 (Orn: ornithine; Pen: penicillamine; SS bond: 2-7) Solubility: H2O Primary Target: mu opioid Primary Target Ki: 0.96nM Molar Mass: 1062.28