The CXCL13 (CXC Ligand 13) protein, also known as B cell attracting chemokine 1 (BCA-1) or B lymphocyte chemo-attractant (BLC) is a chemokine ligand. It is expressed in secondary lymphoid tissue by stromal cells within B-cell follicles and by follicular dendritic cells, macrophages, and germinal center T cells. The CXCL13 receptor CXCR5 is expressed on follicular B cells. CXCL13 interaction with its receptor causes germinal center formation as part of a normal immune response to infection. The main role of CXCL13 is homeostatic immune cell trafficking and levels may become up-regulated under pathological conditions. CXCL13 and CXCR5 are expressed in prostrate, breast, neuronal and oral carcinomas. Elevated serum CXCL13 levels have been observed in lung cancer and breast cancer patients.