FLT1 and neoplasm: Tumor cells release a plethora of factors [59,60], e.g., VEGFR1+—bone marrow-derived progenitor cells that activate VLA-4 that via binding to fibronectin [61,62,63,64] enables entrance to the potential metastatic site and create an appropriate environment, a premetastatic niche that facilitates tumor cells implantation [65,66,67,68,69].