VAV1 and leukemia: Because another leukemia-associated activating STAT5B mutation, STAT5BN642H, produces an expansion of hematopoietic stem cells (HSCs) in the bone marrow (BM) when expressed as a Vav-1–driven transgene in mice (Pham et al, 2018), we performed a comparative analysis of the impact of the Stat5bY665F and Stat5bY665H mutations on early hematopoiesis in the BM (Fig 7).