sequential stages of clinical testing, required by regulatory agencies, used in the development of medical treatments. Pre-clinical testing involves experiments on animals or cells to estimate safety and potential efficacy. Phase I trials are small studies (50-100 subjects) conducted in human beings for the first time to assess safety, pharmacology, or dosing. Phase I studies are usually conducted on healthy volunteers though some are conducted on patients with terminal diseases, such as cancer patients. Phase II trials are larger studies (500 or more subjects) conducted on patients with a disease to assess safety and efficacy and establish a therapeutic dose. Phase III trials are large studies (up to several thousand subjects) conducted on patients to obtain more information on safety and efficacy. Phase IV (or post-marketing) studies are conducted after a treatment has been approved for marketing to gather more information on safety and efficacy and to expand the range of the population being treated.