The Aristaless-related homeobox (Arx) is Mutated in patients with X-linked mental retardation and a range of other neurological diseases. The molecular consequences of these mutations are unclear. Here, we show that two disease-associated mutations disrupt the function of Arx as a transcriptional repressor We found that Arx contains two independent repression domains: an N-terminal octapeptide motif/engrailed homology domain and a novel domain located in the C-terminus. The octapeptide motif functions through interaction with members of the Groucho family of co-repressors. The C-terminal domain functions through interaction with C-terminal binding protein (CtBP). (C) 2008 Elsevier Inc. All rights reserved.