SERPINE1 and acute promyelocytic leukemia: Leukemic promyelocytes contain both u-PA and t-PA.59–61 In addition, as previously described, APL blasts express increased levels of annexin II-associated fibrinolytic activity.38 Additional data suggest that retinoids induce a rapid increase of u-PA activity on APL cell surface, which is promptly down-regulated by an increased production of PA inhibitors, including PAI-1 and PAI-2.32 These mechanisms can contribute to a reduction of fibrinolytic activity in APL cells in response to ATRA.