Estrogen has a rapid action known as nongenomic action. In fact, the definition of nongenomic action does not mean that it is not involve the transcription process. This name originated from the first report of Szege et al., who showed that estrogen rapidly increased cyclic adenosine monophosphate (cAMP) within 15 seconds (Szego and Davis 1967). They suggested that it is too rapid to be accounted for classical genomic action due to estrogen acted through the genomic action requires at least 2 hours after estrogen stimulation However, it is now accepted that the nongenomic can indirectly activate gene transcription via several intracellular signaling pathways (Figure 3–11). In contrast to classical genomic action, nongenomic action is rapidly occurred and required membrane ER. Signaling from membrane ER occurs mainly through the activation of various signal transduction pathways that are often associated with i) G protein coupled receptors (GPCRs), ii) ion channels, and iii) enzyme linked receptors.