PGF and melanoma: The first study to point out the seminal role of TDSFs in pre-metastatic niche formation showed that mice that intravenously inoculated with Lewis lung carcinoma (LLC) or B16 melanoma cells released tumor-derived vascular endothelial growth factor (VEGF) and placental growth factor (PLGF)—molecules involved in angiogenesis—which in turn stimulated BMDCs, specifically VEGFR1+ hematopoietic progenitor cells (HPCs) [11].