Gene-targeting experiments have highlighted the importance of the intracellular protein tyrosine kinases, Lyn, Syk, and Btk, in B-cell receptor-mediated phospholipase C gamma 2 and phosphoinositide 3-kinase activation. In linking such tyrosine kinases with effector enzymes, an important role has emerged for adapter molecules. Adapter proteins nucleate formation of distinct signaling complexes in a specific location within the cell and facilitate the interaction between these signaling components in this particular subcellular compartment, which, in turn, contribute to the qualitative and quantitative control of B-cell signaling.