IntroductionMultipotent hematopoietic stem cells (HSCs) differentiate into precursors with increasingly restricted differentiation potential. This process, called lineage commitment, leads to the generation of oligopotent progenitors and finally to cells that are irreversibly engaged in a unique pathway of differentiation. The identification of progenitors at intermediate stages of differentiation is a fundamental step in understanding lineage commitment. The characterization of such intermediates within the erythromyeloid lineages has identified, in the bone marrow (BM), a granulocytemacrophage-restricted precursor and, more recently, a common myeloid precursor and an erythrocyte-megakaryocyte precursor. 1 T-cell generation occurs in the thymus from hematopoietic precursors present in fetal liver (FL) and adult BM. Although the pathways of intrathymic T-cell differentiation are relatively well characterized, the role of the thymic microenvironment as a unique site for the induction of commitment to the T-cell lineage remains a matter of debate. 2,3 Moreover, the identification of T-cell progenitors before thymic colonization remains incomplete. Cells endowed with a nonrestricted potential of differentiation into T lymphocytes have been phenotypically characterized and isolated from adult BM. Thus, HSCs as well as common lymphoid progenitors (CLPs) were purified. 4,5 The existence of committed T-cell precursors (TCPs) in the BM 6 and more recently in the FL has been suggested. 7,8 However, the identification of surface markers allowing the isolation of these cells has not been achieved. Populations of precursors restricted to the T-cell and natural killer cell (NK) lineages have been reported in fetal thymus, blood, and spleen. [9][10][11][12][13] In 2 independent studies, T and NK precursors have been characterized in fetal thymus either as Fc␥RII/III ϩ9 or as CD90 ϩ NK1.1 ϩ CD117 ϩ , 11 although the total number of precursors present at these sites was not evaluated.We and others have previously reported that the FL, the major hematopoietic organ during embryonic life, provides TCPs with restricted potential of differentiation that continuously colonize the fetal thymus. [14][15][16] In an attempt to identify such intermediate precursors, we isolated different fractions of FL cells and analyzed them in a quantitative manner for lymphocyte precursor activity.Here we identify a novel population corresponding to 0.2% of FL cells that includes the majority (70%) of TCPs in this organ. These cells retained the capacity to differentiate into both T and NK progeny at the single-cell level. When transferred in vivo, they reconstituted the peripheral T and NK compartments and gave rise to intraepithelial T cells of extrathymic origin. This cell population, designated here as common T/NK cell progenitor (C-TNKP), differs both by surface marker and by gene expression analysis from previously described bipotent T/NK precursors present in fetal blood, spleen, and thymus. We propose that these cells represent the immediate ...