The upregulation of VEGF expression by TAM is mediated by CSF-1 and hypoxia-inducing factor (HIF) (28), and is further enhanced in the hypoxic tumor microenvironment, where VEGF attracts monocytes through VEGF receptor (flt-1) (33), thus creating a positive feedback loop of tumor vascularization. This evidence concerns the gene FLT1 and neoplasm.