Hes1 regulates the number and anterior-posterior patterning of mesencephalic dopaminergic neurons at the mid/hindbrain boundary (isthmus)

► Hes1 null mutant mouse embryos display a neural tube closure defect and exencephaly. ► In the mutant midbrain, dopaminergic (DA) neurons are markedly decreased in number. ► In addition, many DA neurons grow dorsally and invade the rostral hindbrain. ► Serotonergic neurons are also ectopically located in the mutant midbrain. ► These anomalies may result from the defective isthmic organizer in Hes1 null mutants.