ITGA4 and progressive multifocal leukoencephalopathy: There is no doubt that ITGA4 is a relevant target in the treatment of multiple sclerosis (MS), but the risk of developing progressive multifocal leukoencephalopathy (PML)10 and the incidence of neutralising anti-natalizumab antibody in up to 50% of the patients11 limits the long term effectiveness of the treatment in these patients.