G-Protein Coupled Receptors (GPCR) sense extracellular signals and activate different Guanine nucleotide binding proteins. Upon activation, the Guanine nucleotide-binding protein G(q) subunit alpha class (GNAQ/GNA11/GNA14/GNA15) can bind to the non-receptor Tyrosine-protein kinase BTK. This binding results in a conformational change in BTK. Subsequently, the structurally modified BTK is released from GNAQ and is now catalytically active. Active BTK can trigger the downstream MAPK p38 pathway. Physiologically, BTK plays a key role in B lymphocyte development, differentiation and signalling.