Interestingly, a myeloid bias in the multi-lineage reconstitution of serially transplanted recipients arose within KI/KI reconstituted mice, reminiscent of aged HSCs. ARAP3 was first purified from porcine leukocytes for its PIP3 binding ability and is highly expressed in hematopoietic tissues, however our studies show that ARAP3 is not a critical cell-intrinsic regulator of hematopoiesis. ARAP3 does not play a cell-autonomous role in regulating HSC homeostasis or function. However, our observation does not rule out a cell-autonomous role for ARAP3 under stress conditions. This phenomenon has been seen in genetic studies of other proteins, such as SIRT1. It is MK-2206 possible that a physiological challenge to the mice is necessary to elicit a specific function for ARAP3 in hematopoiesis. In our model, Arap3 is excised in hematopoietic cells during early LY2109761 development using Vav1-Cre, and compensatory mechanisms may account for normal hematopoiesis in adult mice. It would be interesting to study whether acute loss of ARAP3 in hematopoietic cells of the adult mouse, such as with an inducible Cre system, would reveal a role for ARAP3 in hematopoiesis. Additionally, ARAP3 is part of a dual-GAP family that also includes ARAP1 and ARAP2. Although each family member targets different G-protein substrates and has different active functional domains, these three proteins may have overlapping and redundant roles in hematopoiesis, and may work in conjunction to regulate HSPC function. Thus, genetic studies of mice deficient in Arap1 or Arap2, or with combination deletions of multiple ARAP proteins would clarify whether there is a significant role for the ARAP family of proteins in hematopoiesis. We find that deletion of Arap3 in stromal and osteoblastic cells using Prx1-Cre does not affect steady-state hematopoiesis or HSC homeostasis. Furthermore, deletion of Arap3 in endothelial cells using VEC-Cre does not impact its ability to support wild-type HSCs. However, high efficiency of Arap3 deletion in f/2;VEC mice results in embryonic lethality, suggesting ARAP3 plays an important role in developing endothelial cells, in agreement with previously published data. Since ARAP3 deficiency in embryonic endothelial cells disrupts angiogenic sprouting and vascular structure, this developmental-related dysregulation may indirectly affect HSC emergence, development, and maintenance in the adult mouse. However, our studies do not exclude the possibility that compensatory mechanisms may be upregulated in surviving f/2;VEC mice sometime between HSC emergence and adulthood. One approach to answer this question would be to induce the excision of Arap3 in endothelial cells when ARAP3 is no longer required for vasculature development. Future investigations are warranted to examine the emergence of definitive hematopoietic progenitors and HSCs from the endothelium in the AGM region of Arap3 null embryos and further our understanding of the role of ARAP3 in endothelial cells.