PGF and breast neoplasm: Chaturvedi et al. further implicated hypoxia-induced factors (HIFs) as a trigger for MSC homing to primary tumors, facilitated in part by CCL5/CCR5 and an additional reciprocal MSC-tumor cell feedback loop promoting placental growth factor (PGF) expression by breast tumor cells to reinforce MSC recruitment, angiogenesis, and increased lung and lymph node metastasis (Chaturvedi et al., 2012).