The biological effects of the cysLTs, including LTC4, LTD4, and LTE4 are transduced mainly by a pair of 7-transmembrane receptors, CysLT1 and CysLT2. BAY u9773 is a non-selective antagonist of the CysLT receptors, having about the same IC50 (20-80 nM) for the inhibition of LT responses in a variety of tissue preparations containing either/or both receptors. In contrast, many of the commercially developed CysLT1 receptor antagonists (pranlukast or montelukast) antagonize only CysLT1 receptors. BAY u9773 is thus one of the only available tools for blocking CysLT2 receptors.