Prostaglandins have a diverse range of activities and have a well recognized role in pain and inflammation. Prostaglandin D2 (PGD2) is produced by mast cells, macrophages and Th2 lymphocytes in response to local tissue damage as well as allergic inflammation in diseases such as asthma, rhinitis, and atopic dermatitis. PGD2 binds to a number of receptors, which include the thromboxane-type prostanoid (TP) receptor, PGD2 receptor (DP, also known as DP1) and chemoattractant receptor-homologous molecule expressed on Th2 cells (CRTH2; also known as DP2).