We are interested in the position signaling in the developing nervous system that is responsible for terminal differentiation of neurons. Specifically, we are interested in the roles of a vertebrate retinal cell recognition protein (R-cognin) and those proteins which interact with R- cognin. Is the R-cognin complex at the cell surface a cell signaling mechanism that helps determine the differentiation pathway taken by the embryonic chick retinal neurons? The chick retina provides a useful model system for this because it is large, easily accessible and avascular. We have four specific aims. 1) To confirm the relationship between the chicken multifunctional proteins and R-cognin. 2) To determine the sequence of the R-cognin protein. 3) To determine how R-cognin protein is associated with the retina cell surface. It is already clear that R-cognin has a unique cell surface location in retina tissue, which is likely to be critical for our understanding of how antibody to R-cognin affects developmental increases in GAD and ChAT activity in retina. 4) To determine the cell types in the developing retina that display and synthesize R-cognin. By investigating one specific molecule underlying neurogenesis in retina, we seek a basic understanding of neuronal determination. This is basic research on the establishment and stability of the differentiated state; however, it will help us understand the capacity of the retina for regeneration and provide clues about how abnormal decisions in embryogenesis or early postnatal life might affect the ability of retina to subsequently respond to trauma or which might underlie pathologies in vision that appear later in life.