The mutant mouse staggerer (sg) harbors a deletion within the gene encoding the retinoic acid receptor-related orphan receptor (ROR) . Homozygotes show aberrant cerebellar development. However, the mechanisms responsible for the cerebellar defect are still poorly understood. In the present study, the involvement of neurotrophins (NTs), including nerve growth factor, brain-derived neurotrophic factor, NT-3 and NT-4/5, and their receptors, which play a crucial role in brain development, on the cerebellar defects of sg mice was studied by semi-quantitative reverse transcription-polymerase chain reaction and in situ hybridization histochemistry. An evident alteration of these mRNA levels was observed in both heterozygotes and homozygotes. Such difference was most evident in the internal granule cell layer. Because the changes in NT expressions as well as morphological alterations in sg cerebellum are similar to those in hypothyroid animals, the effect of mutant ROR (RORsg) on transcriptional regulation through the thyroid hormone (TH) response element (TRE) or the ROR response element (RORE) was then studied. RORsg neither activated the transcription through RORE, nor suppressed ROR-induced transcription, indicating that it does not function as a dominant negative inhibitor. On the other hand, although wild-type ROR augmented TH receptor (TR) 1/1-mediated transcription through various TREs, RORsg was not effective in augmenting TR action. These results suggest that the cerebellar defect of the sg mouse is partly caused by the altered expression of NTs and the lack of augmentation of TR-mediated transcription by ROR, as well as the absence of ROR action through RORE.