proteins containing the Gs alpha subunit (i.e. Gαs)-G beta-gammaes (i.e. Gβγ) complex. The complex then dissociate into its Gαs and Gβγ components which act to regulate cell signaling pathways. In particular, Gαs stimulates adenyl cyclase to raise cellular levels of cAMP; cAMP activates PKA, a kinase which in turn activates signaling molecules, in particular, the transcription factor, CREB. Activated CREB stimulates the expression of genes such as c-fos, somatostatin, and corticotropin-releasing hormone that regulate cellular proliferation, cellular differentiation, cellular survival, and angiogenesis. EP4 activation of G proteins also activate PI3K/AKT/mTOR, ERK, and p38 MAPK pathways. Activation of ERK induces expression of EGR1, a transcription factor which controls transcription of genes involved in cellular differentiation and mitogenesis. EP4 also interacts with Prostaglandin E receptor 4-associated protein (EPRAP) to inhibit phosphorylation of the proteasome protein, p105, thereby suppressing a cells ability to activate nuclear factor kappa B, a transcription factor that controls genes coding for cytokines and other elements that regulate inflammation, cell growth, and cell survival (see NF-κB#Structure). The activation of these pathways lead to variety of different types of functional responses depending on cell type, the pathways available in different cell types, and numerous other factors; EP4 activation may therefore have diverse effects on cell function depending on these factors. In many respects, EP4 actions resemble those of another type of another relaxant prostanoid receptor, EP2 but differs from the contractile prostanoid receptors, EP1 and EP3 receptors which mobilize G proteins containing the Gαq-Gβγ complex. Following its activation, EP4 undergoes homologous desensitization. That is, EP4 becomes insensitive to further activation and internalizes. This effect limits the duration and extent to which EP4 can stimulate cells. Agents which activate certain isoforms of protein kinase C can also desensitize EP4 by a process termed heterologous desensitization. Functions Studies using animals genetically engineered to lack EP4 and supplemented by studies examining the actions of EP4 receptor antagonists and agonists in animals as well as animal and human tissues indicate that this receptor serves various functions. However, an EP4 receptor function found in these studies does not necessarily indicate that in does so in humans since EP receptor functions can vary between species. Ductus arteriosus EP4 plays a critical role in postnatal closure of the ductus arteriosus as defined in mice lacking a functional gene for this receptor, i.e. EP4(-/-) mice (see Knockout mouse). About 95% of EP4(-/-) mice die within 3 days of