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Placental growth factor （PlGF-1） is a member of the vascular endothelial growth factor （VEGF） family and is predominantly expressed in the placenta, although it is also expressed at low levels in many other tissues, including the heart, lung, thyroid, liver, skeletal muscle and bone. The human PlGF gene is located on chromosome 14q14 and encodes 4 isoforms of PlGF. The protein is secreted as a glycosylated homodimer and PlGF-1 and -3 are diffusible isoforms whereas PlGF-2 and PlGF-4 have heparin binding domains. Endometrial tissue during the secretory phase of the human menstrual cycle has been shown to secrete PlGF. The main role of PlGF in tissues other than the placenta is angiogenesis in response to pathological ischaemia or injury. In tumour cells, PlGF expression is part of the angiogenic switch that supports tumour vascularisation.