The nuclear orphan receptors for which endogenous ligands have not been identified include nuclear receptor NR0B1 (adrenal hypoplasia congenita critical region on chromosome X gene), NR0B2 (small heterodimer partner), NR1D1/2 (Rev-ErbAα/β), NR2C1 (testicular receptor 2), NR2C2 (testicular receptor 4), NR2E1 (tailless), NR2E3 (photoreceptor-specific NR [PNR]), NR2F1 chicken ovalbumin upstream promoter transcription factor 1 (COUP-TFI), NR2F2 (COUP-TFII), NR2F6 (verbA-related protein), NR4A1 (Nur77), NR4A2 (Nurr1), NR4A3 (Nor1), and NR6A1 (GCNF). Here, NR0B2 is linked to alternating hemiplegia of childhood.