BCR and B-cell chronic lymphocytic leukemia: When the IgM is engaged by antigen (or by another BCR in the case of CLL), the SRC family kinases LYN and spleen tyrosine kinase (SYK) act to phosphorylate tyrosine resides in the cytoplasmic immune-receptor tyrosine-based activation motif domains of CD79a/CD79b [14,15].