FLT1 and melanoma: The lung was the first metastatic site where pioneering PMN research demonstrated that MMP-9 expression in endothelial cells and macrophages was induced by melanoma-secreted VEGF (145) and that FN (α4β1 ligand) was upregulated by VEGFR1+ integrin α4β1+ (VLA-4+) bone marrow-derived HPCs mobilized to the lung via tumor-secreted VEGF-A and placental growth factor (PIGF) (48) (Figure 1).