APJ is a G-protein coupled receptor that is related to the angiotensin receptor. It is primarily expressed in coronary artery endothelium and binds the endogenous ligand, apelin. Apelin signaling through APJ was found to be essential for normal vascular development in frog embryo. APJ can also function as a co-receptor for HIV. APJ and apelin have been shown to be involved in the differentiation of embryonic stem cells toward cardiac lineage.