CXCL13 and breast cancer: Of the chemokine-associated genes in the HRneg/Tneg index, CXCL13 (ligand for the chemokine receptor CXCR5) has been best studied in breast cancer and recently was shown to be the most significantly overexpressed (mRNA and protein) chemokine in a panel of early-stage human breast cancers following a survey of 84 different chemokines [35].