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The functional heterogeneity of hematopoietic stem cells (HSCs) has been comprehensively investigated by single-cell transplantation assay.However,the heterogeneity regarding their physiological contribution remains an open question,especially for those with life-long hematopoietic fate of rigorous selfrenewing and balanced differentiation capacities.In this study,we revealed that Procr expression was detected principally in phenotypical vascular endothelium co-expressing DlI4 and CD44 in the midgestation mouse embryos,and could enrich all the HSCs of the embryonic day 11.5 (E11.5) aortagonad-mesonephros (AGM) region.We then used a temporally restricted genetic tracing strategy to irreversibly label the Procr-expressing cells at E9.5.Interestingly,most labeled mature HSCs in multiple sites (such as AGM) around E11.5 were functionally categorized as lymphomyeloid-balanced HSCs assessed by direct transplantation.Furthermore,the labeled cells contributed to an average of 7.8% of immunophenotypically defined HSCs in E14.5 fetal liver (FL) and 6.9% of leukocytes in peripheral blood (PB) during one-year follow-up.Surprisingly,in aged mice of 24 months,the embryonically tagged cells displayed constant contribution to leukocytes with no bias to myeloid or lymphoid lineages.Altogether,we demonstrated,for the first time,the existence of a subtype of physiologically long-lived balanced HSCs as hypothesized,whose precise embryonic origin and molecular identity await further characterization..