BLK (B lymphoid tyrosine kinase) is a 505-amino acid protein (~56KDa), belongs to the Src non-receptor tyrosine kinases family. Different subcellular localizations of Src-family kinases may be important for the regulation of specific cellular processes such as mitogenesis, cytoskeletal organization and membrane trafficking. Blk is expressed exclusively by B lymphocytes and it is thought to function in a signal transductory pathway specific to this lineage. B lymphoid expression of an active Blk mutant caused proliferation of B progenitor cells and enhanced responsiveness of these cells to interleukin 7. Thus, sustained activation of Blk induces responses normally associated with the pre-BCR.