Progressive multifocal leukoencephalopathy was first recognized as a result of mAb treatment when used in natalizumab-treated patients who had relapsing forms of multiple sclerosis and Crohn disease.20, 21, 22 Since that time, a vigilant assessment of mAbs appears to have been done on the potential for the adverse effect of PML. The gene discussed is PML; the disease is Crohn disease.