Gene expression is critical for the development, patterning, and homeostasis of the organism. Precise temporal and spatial regulation of gene expression at the level of transcription requires a large network of sequence-specific factors, general transcription factors, co-factors, and epigenetic regulators. Malignancies of specific tissues often arise from perturbation of various gene expression levels. Hematopoiesis is one of the most sensitive biological processes to mis-regulation of transcription. To generate all blood cell types from embryonic development throughout the lifetime of the organism, hematopoiesis requires an intricate balance between the maintenance of a permanent stem cell pool and differentiation of multi-potent stem cells into cell types with unique functions. To generate a terminally differentiated, functional immune cell, multiple lineage-restricting steps are involved, with each governed by a specific transcription program. Therefore, gene expression regulation in hematopoietic differentiation is particularly important for an organism to properly develop, maintain oxygen transport to all tissues, and fight against infections. Furthermore, because of detailed understanding of how to isolate cells at different stages and lineages of hematopoietic differentiation, it provides an important model to study the development and differentiation of other adult tissues. Hematopoietic stem cells can be driven to differentiate along three main lineages: myeloid, erythroid, and lymphoid. Despite the discoveries of several transcription factors for specific lineages of hematopoietic differentiation, understanding the gene expression program that allow stem cells to make the decision to initiate lymphoid development still remains incomplete. For example, how is the preinititation complex of transcription (PIC) recruited to the gene promoters? Additionally, how are interactions, if any, coordinated among various sequence-specific factors that were identified via gene-by-gene knockout (KO) approaches? To form the PIC at any gene promoter, transcription factor (TF) IIA, B, D, E, F, and H, and RNA polymerase II (Pol II) must coordinate their promoter-binding and enzymatic activities. TFIID, especially, is important for promoter recognition. As a multi-subunit complex containing TATA-box binding protein (TBP) and 13-14 TBP-associated factors (TAFs), TFIID binds to sequences in the proximal promoter and allows the recruitment of other TFs and Pol II. Previously thought to be invariant from one cell type to another, recently tissue-specific roles for certain TAFs have been uncovered. TAF4B is one of the first TAFs found to have cell-specific expression, since it was identified in human B cells {Dikstein:1996wk}, though a role for its function in hematopoiesis has remained elusive. I used a Taf4b KO mouse line to study its function in both myeloid and lymphoid differentiation. I found that Taf4b KO mice were able to generate myeloid and lymphoid progenitors as well as their wild-type (WT) littermates. Furthermore, both of these types of progenitors from Taf4b KO mice can terminally differentiate into mature cells as well as those from WT mice. Finally, TAF4B-null cells are as competent as heterozygous cells (equivalent to WT in terms of Taf4b expression) to reconstitute the hematopoietic compartment of lethally irradiated mice in all cell lineages tested. In conclusion, TAF4B is dispensable in both myeloid and B cell differentiation. This could be due to TAF4B's high sequence homology with TAF4A. Alternatively, TAF4B can play a role in fine-tuning expression levels of certain B cell or myeloid-specific genes, together with another transcription factor, which cannot be uncovered in a KO mouse approach. I have made a TAF4B-specific polyclonal antibody that can be used to identify its transcriptional targets, as well as identify any potential interaction partners. Though the basal machinery does not seem to play a role in hematopoietic lineage determination, sequence-specific factors have long been implicated in this process. A study using an inducible hematopoietic-specific KO mouse line found that myocyte enhancer factor 2c (MEF2C) is necessary for multi-potent progenitors to differentiate into the lymphoid lineage {StehlingSun:2009df}. Through a candidate approach, I have identified early B cell factor 1 (EBF1) to be a specific interacting partner of MEF2C. Together, they co-occupy and functionally co-activate many B cell specific genes. When MEF2C is depleted in mice, the animals had reduced B cell gene expression as well as increased myeloid gene expression, consistent with MEF2C's role as a lineage fate regulator. I have identified and confirmed several B cell-specific genes that are co-regulated by EBF1 and MEF2C through a genome-wide survey of their binding via chromatin immunoprecipitation followed by exonuclease treatment and deep-sequencing (ChIP-exo). Furthermore, I found that p38 MAPK is the pathway through which MEF2C is phosphorylated and activated to drive B cell differentiation. When phosphorylated, MEF2C prefers to bind its co-activator EBF1, and not its co-repressor HDAC7. Taken together, the results presented in this thesis elucidated the mechanism of activation, binding partners, and downstream targets by which MEF2C is able to regulate lymphoid-specific differentiation. This study contributes to understanding how transcriptional regulation of genes can drive progenitor cells to differentiate down a particular lineage, and provide a novel mechanism for a transcription repressor to switch to an activator during cellular differentiation.