CXCL12 and B-cell chronic lymphocytic leukemia: Additionally, although it is well established that Akt is phosphorylated downstream of CXCL12 signaling in CLL cells [9], it has not been established whether p70S6K, another kinase known to phosphorylate PDCD4 leading to its ubiquitination and degradation, is activated in CLL cells by CXCL12.