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The staggerer (sg/sg) mutation is a spontaneous deletion in the Rora gene that prevents the translation of the ligand-binding domain (LBD), leading to the loss of ROR\alpha activity. The homozygous Rorasg/sg mutant mouse, whose most obvious phenotype is ataxia associated with cerebellar degeneration, also displays a variety of other phenotypes. The heterozygous Rora+/sg is able to develop a cerebellum which is qualitatively normal but with advancing age suffers a significant loss of cerebellar neuronal cells. A truncated protein synthesized by the mutated allele may play a role, both in Rorasg/sg and Rora+/sg. To determine the effects during life span of true haplo-insufficiency of the ROR\alpha protein, derived from the invalidation of the gene, we compared the evolution of Purkinje cell numbers in heterozygous Rora knock-out males (Rora+/-) and in their wildtype counterparts from 1 to 24 months of age. We also compared the evolution of Purkinje cell numbers in Rora+/- and Rora+/sg males from 1 to 9 months. The main finding is that in Rora+/- mice, when only a half dose of protein is synthesized, the deficit was already established at 1 month and did not change during life span....