Embryonic development requires highly specific spatiotemporal control of gene expression to establish diverse cell types. This control is achieved, in part, by RNA binding proteins that regulate the stability and expression of mRNAs. Due to the dynamic nature of RNA regulation and localization in neurons, neurodevelopment is especially sensitive to perturbations in posttranscriptional regulation. The Exon Junction Complex (EJC) is necessary for brain development through supporting the proliferation of neural progenitors; moreover, EJC core components Rbm8a and Magoh are critical for proper development of motor neurons and muscle during embryogenesis in zebrafish. The EJC is a conserved large multiprotein complex deposited on spliced mRNAs that serves as a regulator of mRNA quality control in higher eukaryotes. The EJC can elicit Nonsense-Mediated Decay (NMD) of improperly spliced mRNAs, but also mRNAs containing 3′ Untranslated Region introns (3UIs). The Cancer Susceptibility Candidate 3 (Casc3) protein is a subunit of the conserved exon-exon junction complex (EJC) that appears to potentiate the decay of a subset of transcripts. Moreover, Casc3 has several documented roles in mRNA localization. While the Casc3 gene was first discovered as a factor that is upregulated in metastatic lymph nodes of breast cancer patients, Casc3 is also highly expressed in the nervous system, and expressed throughout development in zebrafish. This proposal aims to address the biological roles and molecular functions of Casc3 in the control of localized mRNA expression in neuromuscular development. I will utilize zebrafish as a model system to investigate the role of Casc3 in neuromuscular development. I will also characterize Casc3 mechanisms of localization and how they are relevant to specific mRNA features and fates in motor neurons. I hypothesize that Casc3 is necessary neuromuscular development, like other EJC core subunits in vivo, and that Casc3 facilitates localization and decay of 3UI-containing mRNAs in motor neurons. We anticipate that Casc3 function extends to other cellular contexts where it is expressed; for example, dissecting Casc3 functions in a neuronal context may provide insight to how its overexpression could contribute to metastatic cancer. Furthermore, this proposed research will enhance our understanding of the mRNA localization function of Casc3 at the molecular level, which will broadly be relevant to its function in these biological contexts.