Capicua is the mammalian ortholog of the drosophilia Cic gene and is part of the HMG-box protein superfamily. Expressed primarily in the fetal brain, Capicua functions as a transciptional repressor and is involved in the development of the nervous system through interaction with the ATXN1 protein. When ATXN1 assembles into stable complexes, it directly binds Capicua, thereby mediating both the activity and expression of Capicua. When Capicua is active, it is able to interact with other developmental proteins to restrict the growth of granule cells and regulate normal neuronal development. Disruptions in the the association of Capicua with proteins such as ATXN1 are thought to cause medulloblastoma, the most common form of perdiatric brain tumor arising from irregular growth of granule cells.