Scl is required for HSC long-term competence. (A) Diagram of the serial competitive transplantation strategy. Scl^+/− bone marrow cells (CD45.2⁺) were mixed with Scl^+/+ competitor bone marrow cells (CD45.1⁺) at 1:1 or 1:5 ratio, and cells were transplanted in congenic irradiated hosts (CD45.1⁺) at 2 Scl^+/− cell equivalents of 4 CRU or 40 CRU. (B-F) Data illustrate results for the 1:1 ratio at 4 CRU cell equivalents (B left panel) and 40 CRU cell equivalents (B-F). The RCA of Scl^+/− cells was calculated and shown as the median ± SEM (*P ≤ .05). (B) Reconstitution by Scl^+/− (CD45.2⁺) and Scl^+/+ (CD45.2⁻) cells after the first transplantation at 4 or 40 CRU equivalents. Reconstitution at 4 and 8 months after transplantation was assessed by flow cytometric analysis of myeloid (CD11b, Gr1) and lymphoid (B220, CD3) cells in the peripheral blood. Data are representative of 2 independent experiments with groups of 5 or 7 mice each. Input Scl^+/− cells before transplantation were 45.7% and 54.5%, respectively. (C-D) Analysis of populations enriched in stem cells and progenitors in the bone marrow. A representative analysis at 4 months of the KSL population is shown (C). The RCA of Scl^+/− cells within the HSCs (Kit⁺Sca1⁺Lin⁻) population and myeloid (Kit⁺Sca1⁻Lin⁻IL7R⁻) as well as lymphoid (CLP) progenitor populations was assessed 4 and 8 months after transplantation (D) (**P ≤ .001). (E) Reconstitution by Scl^+/− cells in the HSC population (KSL) and myeloid and as well as lymphoid progenitor populations (E left panel) or the mature myeloid and lymphoid populations (E right panel) 5 months after secondary transplantation. (F) Thymic reconstitution in primary and secondary transplantation within the CD4⁻CD8⁻ (DN) and CD4⁺CD8⁺ (DP) populations is shown. Comparable results were observed with the CD4⁺ or CD8⁺ populations. All populations shown are Thy1⁺.