FOS and B-cell chronic lymphocytic leukemia: On the other hand, according to Mittal et al., CLL B cells with reduced CTLA-4 expression were characterized by a greater ability to proliferate and survive, a higher expression of STAT1, NFATC2, c-Myc, Ki-67, Bcl-2 molecules, as well as the phosphorylation of STAT1 and c-Fos [40].