Finally, CD4+ and CD8+ T lymphopenia seems to be the primary PML risk factor, following the use of natalizumab (Tysabri; Biogen Idec, Elan Pharmaceuticals), efalizumab (Raptiva; Genentech), and rituximab (Rituxan/MabThera; Genentech, Biogen Idec) for the treatment of multiple sclerosis (MS), Crohn's disease (CD), severe forms of plaque psoriasis, hematologic malignancies, and rheumatoid arthritis (RA) [8]. The gene discussed is CD4; the disease is rheumatoid arthritis.