Asymmetric development of the brain is a conserved feature of vertebrates, and is thought to increase processing capacity by avoiding duplication of function. The best understood model system for asymmetric brain development is the zebrafish. The zebrafish pineal complex includes the pineal organ and the left-sided parapineal organ, which innervates the left habenular nucleus. Our previous works has identified one transcription factor, tbx2b, that is required for parapineal cell specification and left-sided migration (Snelson et al., 2008), but expect that more transcription factors will likely be involved in this process. By identifying more genes and molecules that direct the formation of the parapineal organ, a comprehensive description of how parapineal cells segregate from pineal cells and independently differentiate will emerge. Here, I propose two complementary yet distinct approaches to delineate the gene regulatory circuitry that lead to the development of the asymmetric parapineal neurons. The first is to characterize and identify a novel parapineal-absent mutant, king tut, isolated from a recent genetic screen conducted in the Gamse laboratory. I will perform positional cloning with standard simple sequence length polymorphism (SSLP) genomic markers to identify the gene mutated in king tut. I will also further characterize the king tut phenotypes using cell fate mapping, lineage-specific markers, epistasis experiments, and cell death/proliferation assays to determine why king tut mutants lack a parapineal organ. Secondly, I will focus on constructing a gene regulatory network by identifying transcription factors required for parapineal and pineal development. These aims collectively will not only shed light on the genetic pathways that assign cells to a pineal versus a parapineal fate, but also advance our current understanding for the molecular mechanisms of neuronal diversification, asymmetrical cell migration and left-right asymmetry in the brain.)