Cellular decisions controlling Hematopoietic Stem Cell (HSC) development, self-renewal, and differentiation are thought to be regulated by cytokines that activate the expression of specific transcription factors. For example, mouse knockout studies have demonstrated the importance of SCL/Tal-1 and LMO2 for hematopoiesis during embryogenesis. Other transcription factors, such as GATA-2, are important for both definitive hematopoiesis in fetal liver and to maintain the population of adult hematopoietic stem cells in the bone marrow niche. HSC differentiation and lineage commitment are thought to be related to the expression of PU.1/Spi-1. In addition, signal transduction pathways that culminate in the activation of Pax5 or GATA-1 can promote the differentiation of hematopoietic stem cells into specific B cell or megakaryocyte lineages, respectively.