The cysteinyl leukotrienes, leukotriene C4 , leukotriene D4 and leukotriene E4 , are arachidonic acid derivatives modified by glutathione, Cys-Gly or Cys. Activated mast cells release cysteinyl leukotrienes, which cause smooth muscle contraction, airway constriction, and vascular permeability. The biological effects of the cysteinyl leukotrienes are mediated by two GPCRs, CysLT1 and CysLT2. CysLT2 is expressed in moncytes/macrophages, mast cells and eosinophils. A coding polymorphism in CysLT2 that results in a receptor with reduced response to LTD4 is associated with asthma.