Major conducts research into the neurological diseases including progressive multifocal leukoencephalopathy (PML), caused by JC virus and often found in immunosuppressed patients such as those with HIV/AIDS.
He later moved to the Loyola University Medical School in Chicago, where he also served as Associate Dean of Graduate Programs.
He currently leads a molecular medicine and neuroscience laboratory as a senior investigator in the Division of Intramural Research at the National Institute of Neurological Disorders and Stroke (NINDS) of the NIH.
[13] Major has been quoted extensively in news coverage of the finding that natalizumab (Tysabri) and related monoclonal antibody-based therapies increase the risk of a rare brain disease caused by JC virus.
[14][15][16][17][18][19][20][21] A 2001 Medscape column named Major as a leading expert in white matter brain disease.