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A fundamental scientific principle held for the past 100 years is that children are born with all the neurons they will ever have. New discoveries in stem cell biology and developmental neuroscience are challenging this idea. Because stem cells are selfrenewing and unspecialized, they can generate many different cell types. Neuronal stem cells are found within germinal centers in the adult brain such as the subventricular zone and the dentate gyrus of the hippocampus. Scientists have discovered how to harvest these cells from early embryos or the adult CNS for further cultivation in vitro. A variety of genetic, environmental, and molecular factors can then be studied to determine whether they influence the types of cells produced and whether they grow into neurons, heart, muscle, or other cell types. At present, many of the critical factors that regulate their eventual fates are poorly understood. However, new evidence suggests that genomic repair is one of the critical determinants for maintaining stem cells and mature neurons throughout life. This column reviews studies of the role of genomic repair in embryonic and stem cell neurogenesis and differentiation. Although the interplay between genes and the environment is well established, recent research has found that physical activity can have a striking effect on neurogenesis, neuronal survival, and differentiation in the mature brain. In several studies, voluntary exercise on a running wheel was found to stimulate proliferation of neuronal stem cells in adult rodent dentate gyrus. Physical activity may increase the production of growth factors and other molecules important for neural development. Combinations of several different growth factors have been used successfully to generate dopamineor serotonin-producing neurons from embryonic stem cells in tissue culture. Once these cells have been propagated and partially differentiated, they may be transplanted into the brain for further study. This type of experiment has been successfully used to treat mice with neurodegenerative disorders of the spinal cord and other brain regions. Scientists have also made the startling discovery that stem cells are present in the adult body that have the capacity to differentiate into a range of cell types, including blood cells or neurons. Stem cells are not committed to a specific function, Genetics of Childhood Disorders: XL. Stem Cell Research, Part 4: Neural Horticulture