Acts as a TGF-beta antagonist in the nervous system (By similarity). Exhibits transcriptional repressor activity. Interacts with SMAD2 and SMAD3. Expression is restricted to adult and embryonic central nervous system. Expressed at high levels in the developing cerebellum, ventral metencephalon and myelencephalon at 12.5 dpc (at protein level). In the adult cerebellum, expressed specifically in Purkinje cells. First detected at 11 dpc and expression continues into adulthood with higher levels in embryonic than adult brain. In developing brain, not expressed in neural progenitors during the proliferative phase but expression is detected in postmitotic neural precursors shortly after exiting the cell cycle. Belongs to the SKI family.