CXCR4 and breast cancer: Whilst various populations of metastatic CSCs have been reported, including the CD44+CD24- CSCs in breast cancer (13, 14), C-X-C motif chemokine receptor 4 (CXCR4)+ CSCs in pancreatic ductal adenocarcinoma (PDAC) (12), and CD26+ CSCs in colorectal cancer (11), most of these surface markers are not mechanism-informed and their molecular roles in the pro-metastatic capabilities of CSCs remain poorly understood.