The Groucho (Gro)/transducin-like enhancer of split family of transcriptional 
corepressors are implicated in many signalling pathways that are important in 
development and disease, including those mediated by Notch, Wnt and Hedgehog. 
Here, we describe a genetic screen in Drosophila that yielded 50 new gro 
alleles, including the first protein-null allele, and has two mutations in the 
conserved Q oligomerization domain that have been proposed to have an essential 
role in corepressor activity. One of these latter mutations, encoding an 
amino-terminal protein truncation that lacks part of the Q domain, abolishes 
oligomerization in vitro and renders the protein unstable in vivo. Nevertheless, 
the mutation is not a null: maternal mutant embryos have intermediate 
segmentation phenotypes and relatively normal terminal patterning suggesting 
that the mutant protein retains partial corepressor activity. Our results show 
that homo-oligomerization of Gro is not obligatory for its action in vivo, and 
that Gro represses transcription through more than one molecular mechanism.