CysLT1 (Cysteinyl leukotriene receptor 1) is previous named as LTD4 receptor (leukotriene D4 receptor). It is a receptor for cysteinyl leukotrienes and has highest affinity to leukotriene D4 (LTD4). The receptor mediates contraction and proliferation of smooth muscle, edema, eosinophil migration and damage to the mucus layer in the lung caused by LTD4. A CysLT1 selective antagonist, montelukast, is used clinically in the treatment of asthma.. This response is mediated via a G-protein that activates a phosphatidylinositol-calcium second messenger system. The rank order of affinities for the leukotrienes is LTD4 >> LTE4 = LTC4 >> LTB4.