Linker or adapter proteins provide mechanisms by which receptors can amplify and regulate downstream effector proteins. We describe here the identification of a novel B cell linker protein, termed BLNK, that interfaces the B cell receptor–associated Syk tyrosine kinase with PLCγ, the Vav guanine nucleotide exchange factor, and the Grb2 and Nck adapter proteins. Tyrosine phosphorylation of BLNK by Syk provides docking sites for these SH2-containing effector molecules that, in turn, permits the phosphorylation and/or activation of their respective signaling pathways. Hence, BLNK represents a central linker protein that bridges the B cell receptor–associated kinases with a multitude of signaling pathways and may regulate the biologic outcomes of B cell function and development.