Prostanoids bind to a family of 8 GPCRs to exert their biological effects (Narumiya and FitzGerald, 2001). The protanoid PGE2 causes pain, vasodilation, immunosuppression of T cells, bone resorption and promotion of carcinogenesis. Four related GPCRs, EP1, EP2, EP3 and EP4, each bind to PGE2, but the different G protein coupling status of each receptor leads to distinct biological effects. EP2 couples primarily to Gs to increase intracellular cAMP levels. Mice deficient in EP2 receptor showed impaired ovulation and fertilization, salt-sensitive hypertension (Kennedy et al., 1999). It has been shown that EP2 receptors are also involved in cancer associated immunodeficiency. Thus, genetic knockout of the EP2 receptor reduced tumor growth and prolonged survival in mice that had undergone isograft injection of MC26 or Lewis lung carcinoma cells (Yang et al., 2003). Chemicon's cloned human EP2-expressing cell line is made in the Chem-9 host, which supports high levels of recombinant EP2 expression on the cell surface and contains high levels of the promiscuous G protein to couple the receptor to the calcium signaling pathway. Thus, the cell line is an ideal tool for screening for antagonists of interactions between EP2 and its ligands.