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. This evidence concerns the gene CXCR4 and breast carcinoma.