Treatment with natalizumab, a humanized monoclonal antibody directed against the α4 integrin subunit, demonstrated clinical efficacy in Crohn’s disease,2,3 but was also associated with progressive multifocal leukoencephalopathy,4 presumably resulting from impaired α4β1 integrin-mediated lymphocyte homing to the central nervous system and/or mobilization of CD34+ JC virus-containing stem cells from bone marrow.5 More recently, vedolizumab, a humanized monoclonal antibody specific for the α4β7 integrin heterodimer, demonstrated efficacy in both Crohn’s disease and UC.6,7. The gene discussed is CD34; the disease is Crohn disease.