BTK and B-cell chronic lymphocytic leukemia: AID−/−/Eμ-TCL1 CLL cells had higher levels of BCR signaling, as shown by increased phosphorylation levels of Igα, spleen tyrosine kinase (SYK), and Bruton’s tyrosine kinase (BTK) upon activation with anti-mouse IgM F(ab’)2 as compared to AID-proficient Eμ-TCL1 CLL cells (Fig. 2D).