Angiotensin receptors are a class of G protein-coupled receptors with angiotensin II as their ligands. They are important in the renin-angiotensin system: they are responsible for the signal transduction of the vasoconstricting stimulus of the main effector hormone, angiotensin II. The AT1 and AT2 receptors have a similar affinity for angiotensin II, which is their main ligand. The AT1 receptor is the best elucidated angiotensin receptor. AT2 receptors are more plentiful in the fetus and neonate. Other poorly characterized subtypes include the AT3 and AT4 receptors.