In this paper we propose that in solid tumours such as breast cancer, in addition to genetic alterations such as mutations of VHL [22], PTEN [39], or p53 [40] that are associated with increased levels of HIF1 transcriptional activity, tumour microenvironmental hypoxia may increase CXCR4 expression and thus the metastatic potential of cancer cells. The gene discussed is PTEN; the disease is breast cancer.