In an attempt to further investigate the role of the APRIL–BAFF system in breast cancer, we have concentrated on the role of APRIL and its receptors BCMA and TACI, as APRIL is the ligand found to be differentially expressed in breast cancer (133), and BCMA and TACI were found also in breast cancer cells (149, 151, 152). This evidence concerns the gene TNFSF13 and breast cancer.