The Grg gene encodes a 197 amino acid protein homologous to the amino-terminal 
domain of the product of the groucho gene of the Drosophila Enhancer of split 
complex. Analysis with a polyclonal antisera specific for the Grg protein 
revealed that Grg is a 25 kd nuclear protein that can participate in specific 
protein-protein interactions. A null mutation of the Grg gene was constructed by 
gene targeting. Mice homozygous for this mutation completed embryogenesis and 
were born, but exhibited varying degrees of post-natal growth deficiency. No 
dosage-sensitive genetic interaction was detected between the Notch1 and Grg 
genes in mice heterozygous for a Notch1 mutant allele and homozygous for the Grg 
null mutation.