CXCL12 and cancer: We further analysed mRNA expression of CXCL12 and CXCR4 at the invasive border and cancer centre, finding that CXCL12 expression was significantly increased at the invasive border (P<0.01, Student's t-test; Fig. 5c, right panel), indicating that CXCL12/CXCR4 signalling was significantly upregulated at the invasive border and resulting in gradients of expression between the cancer centre and invasive border.