The cysteinyl leukotrienes (cysLTs; LTC4, LTD4, and LTE4) contract airway and pulmonary vascular smooth muscle, increase vascular permeability, and stimulate mucus secretion, thereby playing a major role in asthma.1,2,3,4 LTC4, LTD4, and LTE4 mediate their actions via at least two receptors designated CysLT1 and CysLT2.1 Cloning of the human CysLT2 receptor reveals it is a 346 amino acid protein with 38% homology to the CysLT1 receptor.5,6 The rank order of binding for leukotrienes to the cloned receptor, as determined using a radioligand binding assay, is LTC4 = LTD4 >> LTE4.5 The mRNA for the human CysLT2 receptor is expressed in lung macrophages, airway smooth muscle, cardiac Purkinje cells, adrenal medulla cells, peripheral blood leukocytes, spleen, placenta, and brain.5,7WARNING This product is not for human or veterinary use.