Multilineage repopulation defect of SCL-heterozygous and SCL-deleted bone marrow. (A) Percentage of donor-derived cells in a competitive repopulation assay. Donor bone marrow cells were obtained from SCL^+/+ (wt, ▪), MxSCL^+/loxP (het, ▦), or MxSCL^-/loxP (deleted, □) mice 6 days after poly(I:C) administration. The proportion of donor-derived myeloid (Mac-1) and B (B220) cells in peripheral blood of recipient mice was assayed at 4 and 16 weeks after transplantation. Mice were killed at 16 weeks to assess donor contribution to T cells (CD4⁺/8⁺ thymocytes) and Lin^neg c-kit⁺ Sca-1⁺ bone marrow cells (LKS). Competitor bone marrow cells were obtained from congenic Ptprca [Ly5.1] mice. Each bar represents the mean and SD of 3 to 5 recipients. The dashed line indicates the expected donor contribution (80%). Comparison of SCL-wild-type and SCL-heterozygous donor cell contribution was performed by a one-sided Student t test. ^*P < .05; ^**P < .01. (B) Southern blot of hematopoietic tissues of a recipient 16 weeks after competitive transplantation with a 4:1 mix of SCL-deleted (MxSCL^-/Δ) and SCL-wild-type competitor cells. The blot was probed with a genomic fragment that can distinguish competitor wild-type (+), loxP targeted (loxP), deleted loxP (Δ), and null (-) SCL alleles. (C) Representative Hb electrophoresis gels of peripheral blood at 16 weeks after transplantation. Competitor cells (20% of transplantation inoculum) were obtained from congenic B6-Hb^dd mice. The β^d (competitor origin) and β^S (donor origin) are indicated. The average erythroid repopulation by donor bone marrow (determined as percentage of total Hb that was β^S) for each population is underneath their respective samples accompanied by the standard deviation. Hb levels were quantified using ImageQuant software analysis. Comparison of SCL-wild-type and SCL-heterozygous donor cell contribution to erythroid cells was performed by a one-sided Student t test. ^**P < .01.