Multiple sclerosis (MS) is a chronic disabling disease of the central nervous system in which the body's immune cells attack and destroy the myelin coating of nerve cells, a process known as demyelination. MS is believed to affect approximately 400,000 people in the United States, striking primarily between the ages of 20 and 50 years, and disproportionately affecting women. In patients who have MS, characteristic lesions can be seen on imaging studies of the brain and spinal cord. These lesions are believed to result when white blood cells migrate into the brain and initiate a series of events resulting in inflammation, demyelination, and plaque formation. Usual care for MS includes drugs that interfere with the overactive immune response such as corticosteroids and other immune-modifying drugs, in particular, interferon beta-1a. In some patients, the interferon is recognized as a foreign protein, triggering the formation of antibodies that bind to it and reduce or eliminate its therapeutic effect.