Tyrosine-Protein Kinase Blk (BLK) contains one protein kinase domain, one SH2 domain and one SH3 domain. BLK is a non-receptor tyrosine kinase, which is involved in B-lymphocyte development, differentiation and signaling. B-cell receptor (BCR) signaling requires a tight regulation of several protein tyrosine kinases and phosphatases, and associated coreceptors. Signaling through BLK plays an important role in transmitting signals through surface immunoglobulines and supports the pro-B to pre-B transition, as well as the signaling for growth arrest and apoptosis downstream of B-cell receptor. Defects in BLK are a cause of maturity-onset diabetes of the young type 11 (MODY11).