Blk is a Src family protein tyrosine kinase expressed in all stages of B cell development . Activation of B cells by various ligands is accompanied by activation of Blk . It has been suggested that Blk is involved in the control of B cell differentiation and proliferation . Blk transcripts have also been detected in human thymocytes, but not in mature T cells, implicating that Blk may play an important role in thymopoiesis . Blk function may be redundant, however, as mice that do not express Blk are not impaired with respect to B cell development and immune response .