CD41, also named GP IIb, is a protein that in human is encoded by the ITGA2B gene. This protein can be associated with GPIIIa to form a heterodimer complex (GPIIb-IIIa) in the presence of Ca2+. This complex can bind one of four different adhesive proteins (i.e,fibrinogen,fibronectin,,von Willebrand factor [Vwf], or vitronectin). CD41 expression has been found on platelets, megakaryocytes, and, more recently, on immature hematopoietic progenitors. CD41 is a reliable marker of early steps of hematopoiesis during ES cell differentiation. CD41 has been used as a marker for megakaryocytic differentiation.