Schematic representation of B-cell receptor signal transduction. Lyn is reciprocally regulated by CD45 and Csk and in turn phosphorylates B-cell antigen receptor (BCR) immunoreceptor tyrosine-based activating motifs (ITAMs) as well as immune tyrosine inhibitory motif (ITIM)-containing immunoreceptors. Positive and negative signals are in turn transmitted by Syk and SHP-1, respectively. Yellow bands on Igα and β chains represent ITAM domains. Orange bands on CD22 and FcγRIIb represent ITIM domains. MAPK, mitogen-activated protein kinase; PLCγ2, phospholipase C γ2.