We have isolated cDNAs representing multiple members of murine groucho 
homologues, designated Grg for groucho-related genes. Among them, Grg3 appears 
to produce two transcripts. One of the Grg3 transcripts contains coding sequence 
for a complete Groucho protein homologue. The second transcript contains coding 
sequence for only the two amino-terminal domains of the Groucho protein, 
followed by a hydrophobic tail and a stop codon. We analyzed the expression of 
both transcripts in mouse embryos using RNase protection and in situ 
hybridization. Expression was detected during cell determination in the nervous 
system and in somitic mesoderm, overlapping Notch1 expression and adjacent to 
Mash1, MyoD and Myf5 expression. Thus, the expression pattern of Grg3 suggests a 
conserved role in the Notch signalling pathway to regulate expression of basic 
helix-loop-helix proteins and cell determination. Grg3 expression was also 
consistently detected in epithelial structures undergoing mesenchyme induction.