We have recently discovered that CABIN1, a component of the HIRA histone H3.3 chaperone complex, is necessary for cranial neural crest cell migration and development of craniofacial cartilages in zebrafish. The HIRA chaperone complex is responsible for incorporating the histone H3 variant, H3.3, into nucleosomes localized to chromatin that is in an open configuration associated with active transcription [2, 3]. Interestingly, craniofacial defects are also observed in zebrafish with dominant negative mutations in histone H3.3 [4]. Together, these results suggest that regulated incorporation of histone H3.3 into the chromatin of developing cranial neural crest cells is essential for changes in gene expression associated with craniofacial development. The long-term goal of this research is to elucidate the regulation of histone H3.3 chromatin incorporation by the in the etiology of craniofacial malformations. The overall objective of this R03 application is to identif key cellular processes and molecular targets of CABIN1 function during cranial neural crest cell differentiation. Our central hypothesis is that CABIN1 regulates craniofacial development by preventing premature expression of genes involved in the chondrogenic differentiation of cranial neural crest cells. This hypothesis is based on 1) published data regarding CABIN1 interacting proteins discovered in developing muscle cells and T-cells 2) the known roles of these same proteins in cranial neural crest development, and 3) our preliminary data showing that CABIN1 gene knockdown retards migration of cranial neural crest in zebrafish and leads to craniofacial malformations. The rationale underlying the proposed research is that establishing the cellular and molecular function of CABIN1 will enable discovery of novel epigenetic mechanisms that regulate craniofacial development. It is expected that the results of these studies will not only contribute to our understanding of the etiology of 22q11.2 deletion syndromes, but also expand our knowledge of the pathways generally affected in disorders of cranial neural crest origin. To attain the goal of this application, we plan to objectively test our central hypothesis using primary cultures of embryonic mouse cranial neural crest cells to pursue the following two specific aims: Aim 1. Identify transcriptional targets of CABIN1 in cranial neural crest cells. Based on its association with the HIRA complex and its role as a transcriptional co-repressor [8, 13], our working hypothesis is that CABIN1 re- presses HIRA-mediated activation of genes essential for craniofacial development. Aim 2. Establish the cellular role of CABIN1 in cranial neural crest development. Based on the known role of CABIN1 in developing muscle cells [8], our working hypothesis is that CABIN1 inhibits the premature differentiation of cranial neural crest cells during migration. The proposed studies are significant and innovative because they will critically advance our understanding of CABIN1 interactions in the HIRA complex and its role in the gene regulatory networks governing craniofacial development, the dysregulation of which lead to craniofacial birth defects.