PROJECT SUMMARY/ABSTRACT Limb development is strictly regulated by gene regulatory networks governed by a complex network of regulatory elements, transcription factors, and epigenetic modifiers. Each component is interconnected, and misregulation at any point in the developmental timeline can lead to congenital limb defects, which affect 1 in 2,000 newborns. One factor that regulates early limb initiation and outgrowth in both mice and zebrafish is PRDM1, though the mechanism is unknown. Uncovering this has become increasingly more important as we have recently identified three human families with split hand/foot malformation (SHFM) of unknown genetic etiology but potentially pathogenic PRDM1 variants. PRDM1 is a versatile transcription factor, capable of both activation and repression by binding directly to DNA at its zinc finger domain or by recruiting chromatin modifiers to its SET domain to regulate transcription . The DNA-binding domain of PRDM1 is either absent or mutated in zebrafish mutants and in SHFM families. Therefore, I hypothesize that direct DNA binding through the zinc finger domain of PRDM1 is necessary for activation of gene expression required for maintaining AER activity during limb development. The proposed study will test this hypothesis in three focused aims. The first aim will use a conditional rescue experiment to determine the functionally active domain of Prdm1a (zebrafish ortholog) during limb development. Constructs of modified prdm1a, in which different domains are deleted, will be injected into null zebrafish mutants. The second aim will determine where Prdm1a binds to DNA during pectoral fin bud development. The third aim will test the pathogenicity of each SHFM PRDM1 variant in zebrafish following mutagenesis by CRISPR-Cas9 and homology directed repair. I will analyze pectoral fin bud development and DNA binding in each mutant. Together, the results from this study will improve our understanding of one of the many pathways involved in limb development. It will also provide insight into how mutations in PRDM1 can lead to congenital limb disorders and help us better predict disease outcomes or phenotype severity based on the given variant.