In this proposal, I seek to characterize the convergence of retinoic acid (RA) and pou2 signaling on hindbrain patterning and mauthner cell formation. A putative low-to-high RA concentration gradient between the mid-hindbrain boundary (MHB) and the hindbrain/spinal cord boundary (HSB) might function to specify posterior hindbrain fates. Therefore, I describe a series of experiments to test whether pou2 is required in the RA pathway leading to supernumerary mauthner neurons in r2. As decades of work have shown, RA is required for many developmental processes and either excess or deficiency can cause severe birth defects including teratocarcinomas. Thus, analysis of developmental signaling networks in zebrafish will have relevance to many human diseases, including cancer.