In the liver, TGF-β1 is a major inducer of hepatocyte polyploidization (9) and polyploid megakaryocytes are the primary source of TGF-β1 in patients with primary myelofibrosis (10), suggesting the existence of a regulatory loop between fibrosis and polyploidization (10). Here, TGFB1 is linked to myelofibrosis.