The ability to make functionally competent brain precursor cell lines may have important consequences for a number of outstanding questions in mammalian CNS development. The diversity of neurons is one of the most striking conclusions of recent molecular studies of the nervous system. In our work we provided some of the earliest data to support this conclusion.30-32 Our preliminary data with precursor cell lines suggest that these cells give neurons with highly differentiated characteristics. If further work continues to support this conclusion, then immortal cell lines will be useful in analyzing the mechanisms that make neurons different. The methods that can be applied to these cell lines include manipulation in tissue culture, transplantation into the developing CNS, and application of the methods of mammalian cell genetics to link gene function to neuronal specificity.