BIRC5 and B-cell chronic lymphocytic leukemia: Resistance to apoptosis is a hallmark of CLL disease and it is induced by intrinsic genetic lesions and extrinsic signals from microenvironmental elements that cooperate to keep active intracellular pro-survival pathways such as PI3K-Akt signaling or up-regulate anti-apoptotic molecules such as BCL-2, BCL-XL and survivin.