Syk is a cytoplasmic protein tyrosine kinase that translocates to the plasma membrane upon B cell antigen receptor (BCR) or the high-affinity IgE receptor (FcepsilonRI) triggering, and phosphorylates downstream adaptor proteins, thereby providing docking sites for initiation of subsequent signaling pathways, such as calcium mobilization, cytoskeleton remodeling, or transcription of specific genes. Syk binds to the receptor assemblies through interactions of its pair of SH2 domains with ITAM motives of the receptor, which have been phosphorylated by Src-family kinases. These kinases also help to activate Syk by phosphorylation of its activation loop.