CD42b, encoded by the GP1BA gene, is a surface membrane glycoprotein expressed on the membrane of megakaryocytes, splenocytes and platelets.  CD42b forms a part of the CD42a-d complex (also called the glycoprotein Ib-IX-V receptor) and is the surface platelet receptor for von Willebrand factor (vWF) on endothelium.  CD42b is useful in differentiating acute megakaryoblastic leukemia subtype M7 (AML-M7) that are typically positive for CD41, CD42b and CD61 from acute myelofibrosis, which are CD42b negative.