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During forebrain development, a telencephalic organizer called the cortical hem is crucial for inducing hippocampal fate in adjacent cortical neuroepithelium. How the hem is restricted to its medial position is therefore a fundamental patterning issue. Here, we demonstrate that - interactions are crucial for the formation of the hem. Loss of either gene causes a region of the cortical neuroepithelium to transform into hem. We show that FOXG1 regulates expression in the cortical primordium. In the absence of , the presence of is sufficient to suppress hem fate, and hippocampal markers appear selectively in -expressing regions. FOXG1 also restricts the temporal window in which loss of results in a transformation of cortical primordium into hem. Therefore, and form a genetic hierarchy in the spatiotemporal regulation of cortical hem specification and positioning, and together ensure the normal development of this hippocampal organizer.