Analysis of neural cell lineage in vertebrates has been limited by a lack of methods for introducing stable tracers into individual cells at relatively late stages of development. Recent progress in the design of recombinant retroviral vectors provides a novel approach to this problem. When a retrovirus infects a dividing cell, its genome integrates into a chromosome of the infected cell and is inherited by that cell's progeny. For lineage tracing, viral structural genes are replaced by a bacterial beta-galactosidase gene; infected cells are therefore unable to produce new virions, but can produce galactosidase, which is detectable histochemically. By infecting cells and identifying their progeny at appropriate stages, it has been possible to obtain new data on cell lineage in retina, cerebral cortex, optic tectum, and peripheral nerve.