The leukotriene receptors fall into two groups of structurally related GPCRs: the chemoattractant-like BLT receptors and the CysLT receptors, which are related to the family of nucleotide-binding receptors. Of the four cloned leukotriene receptors, BLT1 and BLT2 receptors have highest affinity for the leukotriene LTB4 with BLT1 binding LTB4 with much higher affinity as compared to BLT2. The BLT receptors are predominantly expressed in the peripheral blood including leukocytes, granulocytes, macrophages, and eosinophils. The CysLT1 and CysLT2 receptors in contrast each bind the cysteinyl leukotrienes LTC4 and LTD4 with high affinity. CysLT receptors are expressed in peripheral blood leukocytes, spleen, and lung as well in a number of other tissues at lower levels. The CysLT1 receptor has been implicated as an important mediator in asthma, and antagonists of this receptor such as monteleukast have been used clinically in the treatment of asthma.