FLT1 and neoplasm: In a pioneering paper that emphasized the significance of immune cells in the formation of the (pre-)metastatic niche, Kaplan and colleagues demonstrated that VEGF receptor 1-expressing (VEGFR1+, also known as FLT1) bone marrow-derived cells (BMDCs) are recruited to the lung in response to tumor-derived VEGFA and placental growth factor before the arrival of transplanted tumor cells (Kaplan et al., 2005).