The development of mature blood cells from hematopoietic stem cells is precisely regulated. Expression of cytokine receptors, survival programs, and other key proteins provide the means for individual cells to survive, proliferate, differentiate and/or function. Control of transcription of genes is responsible for these cellular programs is one level of regulation. PU.1, and ets family member, is expressed in primitive hematopoietic cells and is up-regulated during myeloid and B cell development. Without PU.1 there is no development of B cells, mature dendritic cells and monocytes/macrophages and no terminal differentiation of neutrophils. Therefore, how do transcription factors, such as PU.1, assemble with specific transcriptional partners to promote commitment and maturation of many different lineages? Furthermore, must PU.1 be continuously expressed to maintain neutrophil dendritic cell and monocyte/macrophage differentiation? To define the mechanisms by which PU.1 promotes myeloid and lymphoid lineage commitment and maturation we propose to address in our Specific Aims: 1. Which PU.1 subdomains within the transactivation and PEST domains promote myeloid and lymphoid development in vivo? In this Specific Aim we will explore the molecular requirements for various subdomains by restoration of freshly isolated PU.1 null hematopoietic cells with PU.1 mutants and then evaluate gain of specific gene expression and cell function. Selected PU.1 mutants will then be assessed for their regulation of development in vivo. 2. What are the roles of PU.1 in the development of myeloid and lymphoid dendritic cells? We will elucidate the cellular and molecular mechanisms responsible for the in vivo less of both myeloid and lymphoid dendritic cells. We will also evaluate the roles of PU.1 subdomains in dendritic cell development. 3. Must PU.1 expression is maintained during and after neutrophil and monocyte development? We will examine the requirement for PU.1 after development ensures and in mature cells by using conditional PU.1 expressing mice that are homozygous for the PU.1 null allele. Completion of these proposed studies will help to elucidate molecular and cellular mechanisms governing hematopoietic development.