How does the complex array of cell types and functions in the mammalian brain develop? Tracking cells by gene expression shows how their fates derive from organization within the simple embryonic neural tube. While forming functional domains in the developing brain, cells crawl considerable distances to reach their final destinations. So it has been extremely difficult to determine when and where during this process the neuronal cells 'learn' which distinct brain region they are fated to become. But advances in mouse genetic manipulation now allow researchers to indelibly mark a cell population at a discrete place and time in development, based on its gene-expression pattern, and to follow the cells' subsequent progress. A series of papers have exploited these innovations to trace the 'decisions' of cells to form certain brain centres (nuclei) back to instructions in the developing neural tube in the embryo.
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