as efalizumab, belatacept, and various transplant drugs, which are meant to weaken the immune system. Multiple sclerosis medications Natalizumab (Tysabri) was approved in 2004 by the FDA for the treatment of multiple sclerosis (MS). It supposedly works by preventing white blood cells from entering the brain. It was subsequently withdrawn from the market by its manufacturer after it was linked with three cases of PML. All three initial cases were taking natalizumab in combination with interferon beta-1a. After a safety review, the drug was returned to the market in 2006 as a monotherapy for MS under a special prescription program. As of May 2011, over 130 cases of PML had been reported in MS patients, all in patients who had taken natalizumab for more than a year. While none of them had taken the drug in combination with other disease-modifying treatments, previous use of MS treatments increases the risk of PML between three- and four-fold. The estimated prevalence of PML in MS is 1.5 cases per thousand natalizumab users. Around 20% of MS patients with PML die, and most of the rest are very disabled. One case study describes an MS patient who, during a 4-year course of dimethyl fumarate, developed PML and died. Fingolimod (Gilenya) was approved in 2010 by the FDA for MS. In 2015, the first case of PML and a case of "probable PML" were reported by two Gilenya users that could not be tied to previous immunosuppressant therapies. These new cases are now being added to the drug information sheet included with every prescription (i.e. the "drug label"). Pathogenesis PML is a demyelinating disease, in which the myelin sheath covering the axons of nerve cells is gradually destroyed, impairing the transmission of nerve impulses. It affects the subcortical white matter, particularly that of the parietal and occipital lobes. PML destroys oligodendrocytes and produces intranuclear inclusions. It is similar to another demyelinating disease, MS, but progresses much more quickly. The breakdown of myelin is proportional to the degree of immunocompromise. Diagnosis PML is diagnosed in a patient with a progressive course of the disease, finding JC virus DNA in spinal fluid together with consistent white-matter lesions on brain magnetic resonance imaging (MRI); alternatively, a brain biopsy is diagnostic when the typical histopathology of demyelination, bizarre astrocytes, and enlarged oligodendroglial nuclei are present, coupled with techniques showing the presence of JC virus. Characteristic evidence of PML on