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Previous studies revealed that mAb BB9 reacts with a subset of CD34⁺ human BM cells with hematopoietic stem cell (HSC) characteristics. Here we map BB9 expression throughout hematopoietic development and show that the earliest definitive HSCs that arise at the ventral wall of the aorta and surrounding endothelial cells are BB9⁺. Thereafter, BB9 is expressed by primitive hematopoietic cells in fetal liver and in umbilical cord blood (UCB). BB9⁺CD34⁺ UCB cells transplanted into nonobese diabetic/severe combined immunodeficient (NOD/SCID) mice contribute 10-fold higher numbers of multilineage blood cells than their CD34⁺BB9⁻ counterparts and contain a significantly higher incidence of SCID-repopulating cells than the unfractionated CD34⁺ population. Protein microsequencing of the 160-kDa band corresponding to the BB9 protein established its identity as that of somatic angiotensin-converting …