However, due to the ubiquitous expression of VCAM-1, systemic blocking of the VCAM-1 homing cascade was associated with severe adverse events like progressive multifocal leukoencephalopathy (PML) (28, 29), underscoring the need for gut-selective targeting of T cell trafficking. This evidence concerns the gene VCAM1 and progressive multifocal leukoencephalopathy.