To decouple the effects of VLA-4/VCAM-1 interactions from cytokine secretion on gap formation in endothelial monolayers, we performed experiments with K562 human erythroleukemia wild type (WT) cells and K562 cells stably transfected with the α4 integrin (K562 VLA-4). This evidence concerns the gene VCAM1 and erythroleukemia.