The combinatorial interaction among transcription factors is believed to determine hematopoietic cell fate. Stem cell leukemia (SCL, also known as TAL1 [T-cell acute lymphoblastic leukemia 1]) is a tissue-specific basic helix-loop-helix (bHLH) factor that plays a central function in hematopoietic development; however, its target genes and molecular mode of action remain to be elucidated. Here we show that SCL and the c-Kit receptor are coexpressed in hematopoietic progenitors at the single-cell level and that SCL induces c-kit in chromatin, as ectopic SCL expression in transgenic mice sustains c-kit transcription in developing B lymphocytes, in which both genes are normally down-regulated. Through transient transfection assays and coimmunoprecipitation of endogenous proteins, we define the role of SCL as a nucleation factor for a multifactorial complex (SCL complex) that specifically enhances c-kit promoter activity without affecting the activity of myelomonocytic promoters. This complex, containing hematopoieticspecific (SCL, Lim-only 2 (LMO2), GATA-1/ GATA-2) and ubiquitous (E2A, LIMdomain binding protein 1 [Ldb-1]) factors, is tethered to DNA via a specificity protein 1 (Sp1) motif, through direct interactions between elements of the SCL complex and the Sp1 zinc finger protein. Furthermore, we demonstrate by chromatin immunoprecipitation that SCL, E2A, and Sp1 specifically co-occupy the c-kit promoter in vivo. We therefore conclude that c-kit is a direct target of the SCL complex. Proper activation of the c-kit promoter depends on the combinatorial interaction of all members of the complex. Since SCL is down-regulated in maturing cells while its partners remain expressed, our observations suggest that loss of SCL inactivates the SCL complex, which may be an important event in the differentiation of pluripotent hematopoietic cells. Tissue-restricted members can regulate gene expression by binding to E-box DNA sequences (CANNTG) following heterodimerization with ubiquitously expressed E2A gene products (E12 and E47) or HEB. Different dimers exhibit preferential binding to specific E-boxes, and this selectivity is thought to be an important determinant in the spatio-temporal control of gene expression. SCL is a prototypic tissue-specific bHLH factor normally expressed in pluripotent hematopoietic precursors, vascular endothelial cells, and the central nervous system (reviewed in Begley and Green 2 ) and acts as a master regulator of hematopoietic development. Indeed, SCL Ϫ/Ϫ mice lack all primitive and definitive hematopoietic lineages and precursors. 3,4 Complementary-gain-of-function experiments in zebra fish and Xenopus have demonstrated that SCL plays a role in specifying the formation of hemangioblasts, the common precursors of vascular endothelial and hematopoietic stem cells. [5][6][7] As for many hematopoietic transcription regulators, the SCL gene was originally identified by virtue of its involvement in a tumor-specific translocation. 2 In fact, chromosomal rearrangements causing aberrant activation of...

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