The role of the 5-HT innervation of the cerebellum in locomotor control is unknown but interestingly: (i) all levels of the cerebellum receive 5-HT fibers; (ii) 5-HT modulates the electrophysiological activity of many cell types in the cerebellum; (iii) the level of cerebellar 5-HT increases during motor activity; and (iv) 5-HT cell metabolism abnormalities were detected in cerebellar ataxia (for review Trouillas, 1993; Schweighofer et al., 2004). This evidence concerns the gene HTR5A and cerebellar ataxia.