Nurr1 belongs to a subfamily of orphan nuclear receptor transcription factors which also includes NGFI-B and Nor1. In situ hybridization demonstrates partly overlapping and partly unique cellular expression patterns in the developing and adult nervous system. These patterns will be reviewed. All three factors can bind to DNA as monomers and cause constitutive transcription. Interestingly, Nurr1 and NGFI-B, but not Nor1 can also heterodimerize with RXR. In such heterodimers, RXR can be activated by a vitamin A ligand. The functional and neuroanatomical studies therefore indicate possible redundancy between members of this subfamily of the steroid/thyroid hormone receptor superfamily. Midbrain dopamine (DA) neurons, however, express only Nurr1, making redundancy impossible in this region. Moreover, Nurr1 mRNA is found in the developing mesencephalic flexure area, prior to any known marker of DA neurons. When mice were generated in which functional Nurr1 mRNA expression had been eliminated, we found a complete absence of DA neurons. Detailed studies of Nurr1 -/- mice, using a host of markers for the DA neuron phenotype supports the conclusion that these neurons fail do be formed in the absence of the transcription factor. Nurr1 -/- mice die within 24 hours after birth, and appear unable to suckle and feed themselves. The exact mechanism behind this early postnatal lethality remains to be explained. Although the CNS and peripheral organs appear grossly normal in the knockouts, we currently favor the hypothesis that the animals succumb so soon after birth for other reasons than the lack of the nigrostriatal DA system.