Acute Radiation Syndrome (ARS) (sometimes known as radiation toxicity or radiation sickness) is an acute illness caused by irradiation of the entire body (or most of the body) by a high dose of penetrating radiation in a very short period of time (usually a matter of minutes). The major cause of this syndrome is depletion of immature parenchymal stem cells in specific tissues. Caused by a critical exposure to radiation, acute radiation syndrome includes neutropenia, nervous system damage, and pneumonia with high fatality rate.

The US Food and Drug Administration (FDA) has granted Orphan Drug Designation to EC-18 for the treatment of Acute Radiation Syndrome (ARS).