Decades of uncertainty preceded the correct pharmacological classification of prostanoid receptors in the 1980’s. Yet by the early 1990’s all of the receptors described by the pharmacological classification had been structurally verified by molecular biological studies. Prostaglandins D₂, E₂, F_2a, I₂, (PGD₂, PGE₂, PGF_2a, PGI₂) and thromboxane A₂ (TxA₂) are currently believed to exert their effects by interacting with distinct subsets of receptors that preferentially recognize each of these individual prostanoids: these receptor classes are designated DP, EP, FP, IP, and TP, respectively. PGE₂ alone is the only prostanoid conclusively proven to preferentially interact with more than one PGE₂-sensitive receptor. These receptor subtypes have been designated EP₁, EP₂, EP₃, and EP₄.