A naturally occurring chemical that performs a wide range of biological roles. It is involved in the laying down of memories in the BRAIN; in killing viruses, bacteria and cancer cells; and in helping to control blood pressure. It is a muscle relaxant and is important in maintaining the heart and circulation in good condition. NO is also the toxic agent released by macrophages (see MACROPHAGE) to kill invading germs and spreading cancer cells. It acts as an essential NEUROTRANSMITTER and protects nerve cells against stress. Researchers are studying how it might be used to treat diseases, for example by using it as an inhaled gas in certain respiratory conditions.