We show here for the first time that the ligand for CXCR5, CXCL13, is the most significantly overexpressed chemokine in breast cancer, supporting the idea of a role of CXCL13/CXCR5 interactions in promoting initiation and/or progression of this tumour type and, possibly, other human cancers. This evidence concerns the gene CXCR5 and breast cancer.