B cells represent a critical component of the immune system: in the absence of these cells, the ability to fight infections and to establish long-term protective immunity are severely impaired. B cell development and function depend on elaborate mechanisms that tightly control gene expression and the physiologic breakage and repair of DNA that occurs uniquely in lymphoid progenitors. Although these mechanisms function with remarkable fidelity, they can be perturbed by discrete genetic lesions, leading to immune dysfunction or lymphocytic cancers in humans. Thus, dissecting the molecular pathways that control B cell development is critical to advancing our understanding of normal immune system physiology and diseases that arise from the B cell compartment. The overarching objective of our proposal is to gain new insight into the mechanisms that globally control gene expression during B cell development. It has been established that a regulatory network anchored by several DNA-binding proteins is primarily responsible for controlling gene transcription during B cell development. This network enforces widespread gene repression as well as activation, thus imparting a B- lineage identity onto developing progenitors. Yet, the factors that function downstream of B-lineage DNA- binding proteins to globally orchestrate gene repression and activation have only begun to be identified. In our previous studies, we identified a novel mutant mouse strain named Justy (for just T cells), which carries a recessive mutation that abolishes B cell development but does not impair other major aspects of mouse physiology. The causative lesion is a point mutation that dramatically reduces, but does not abolish, expression of a protein called Gon4-like, which contains homology to factors that mediate epigenetic regulation of gene transcription. Gene expression profiling indicated that decreased Gon4-like expression does not impair activation of B-lineage genes, but does prevent the repression of genes encoding proteins that can antagonize B cell development. Among these are genes encoding factors that have lineage-instructive roles in hematopoietic developmental pathways. Based on these data, we hypothesize that Gon4-like is a critical component of the regulatory network that establishes a B-lineage gene program identity while extinguishing alternative lineage programs, particularly those associated with myeloid cell development. We propose to employ mouse genetic tools, cell culture systems, retroviral gene transfer technology and methods for quantifying gene expression to carry out studies that will integrate Gon4-like into the regulatory circuitry the controls B cell development. To complement these studies, we will use molecular and biochemical approaches to identify co-factors for Gon4-like and to begin defining the role of Gon4-like in gene regulation on a genome-wide scale. Completion of these studies will break new ground in our understanding of the mechanisms that control hematopoietic lineage fate decisions and imprint commitment to a B cell fate. Such knowledge will provide novel insights regarding the relationships between impairment of B cell development and the appearance of immunodeficiencies or B cell progenitor-derived lymphoid cancers.