ELF3, a member of the ETS transcription factor family, has been shown to transactivate the transforming growth factor beta type II receptor (TGF-betaRII) promoter. Previously we showed that Elf3-null mice have a defect in the small intestine caused by a failure of small intestinal epithelial cells to differentiate and that these cells produced significantly lower levels of Tgf-betaRII. To prove that the defect observed in Elf3-null mice resulted from the lack of Elf3-dependent activation of Tgf-betaRII expression, we performed a genetic rescue.