CXCL12 and B-cell chronic lymphocytic leukemia: In lymphoid organs, circulating CLL cells are surrounded by a supportive microenvironment including antigens (e.g., CD40L, B-cell receptor/BCR, VCAM-1, CD49d), cytokines (e.g., IL-6, IL-10) and chemokines (e.g., CXCL12, CXCL13) and extracellular matrix proteins (e.g., fibronectin) provided by other CLL cells, T and NK cells, monocyte-derived nurse-like cells and stromal cells, all of which provide additional survival and anti-apoptotic signals to CLL cells [2,66,67,68,69,70,71].