BTK is expressed in all cell lineages of the hematopoietic system, except for T cells. Therefore BTK expression is found in B lymphocytes, platelets, erythroid and myeloid cells (monocytes, macrophages, granulocytes and dendritic cells). Within the B cell lineage, BTK is already expressed in the earliest detectable B cell precursors, and expression is downregulated in plasma cells.