['The transcription factor SOX9 has been shown previously to have an essential role in the differentiation of a small number of discrete cell lineages. In the intestine, Sox9 is expressed in the epithelial cells of the crypts and is a target of Wnt signaling.', 'To examine the function of SOX9 in the intestine, we inactivated the Sox9 gene in intestinal epithelial cells by generating mice that harbored a conditional Sox9 gene and a Villin-Cre transgene.', 'In the absence of SOX9, Paneth cells were not formed, but the differentiation of other intestinal epithelial cell types was unaffected. The lack of SOX9 also lead to crypt enlargement, to a marked increase in cell proliferation throughout the crypts, and to replacement of the Paneth cells by proliferating epithelial cells.'] Is sOX9 required for the differentiation of paneth cells in the intestinal epithelium?