FLT1 and metastatic neoplasm: Non-malignant bone marrow-derived haematopoietic progenitor cells (BMDCs) expressing the vascular endothelial growth factor receptor-1 (VEGFR-1) precede the arrival of even single metastatic tumour cells and macro-metastatic disease at distant sites, according to early work by Kaplan and colleagues defining the pre-metastatic niche and its temporal and functional relationship to metastasis.