Lymphoid organ development results from a complex interplay of genetically controlled molecular and cellular networks. While most of the molecular mechanisms underlying lymphoid organogenesis remain unknown, recent findings demonstrate that homeodomain transcription factors are essential to these processes. One such homeoprotein is Pbx1 that binds DNA with a subset of Hox proteins, including the Hox11 oncoprotein aberrantly accumulated in T-cell acute leukemia. Like Hox11, Pbx1 was originally isolated as a proto-oncogene in pre-B-cell leukemia. By using knockout mouse models, cellular and biochemical approaches, we demonstrated that Pbx1 and Hox11 loss of function mutations result in reduced spleen mesenchymal proliferation accompanied by spleen agenesis. Furthermore, we established a genetic network regulating spleen mesenchymal expansion and demonstrated the essential role of the Pbx1-Hox11 transcriptional pathway during spleen ontogeny. Although a few Pbx1- and Hox11-targets have been identified, the growth-related transcriptional networks orchestrated by these homeoproteins during lymphoid organogenesis remain mostly unknown. Our research objective is to perform a comprehensive search for direct Pbx1 and Hox11 growth-related target genes and de-convolute the global gene regulatory network governed by these homeoproteins during lymphoid organogenesis. We plan perform a comprehensive analysis to uncover genes differentially expressed in wild type and single Pbx1 and Hox11 mutant embryonic spleens. Next, our studies will focus on those direct downstream targets that are likely responsible for the cellular proliferation/differentiation defects observed in these mutant spleens. We will validate these potential targets by performing in situ hybridization and immunohistochemistry analysis on embryonic spleen sections and further corroborate whether Pbx1 and/or directly control these targets by chromatin immunoprecipitation and transcriptional assays.