BACKGROUND & AIMS: Stem cells within the intestinal epithelium generate daughter cells that undergo lineage commitment and maturation through the combined action of the Writ and Notch signaling cascades. Both pathways, in turn, regulate transcription factor networks that further define differentiation toward either enterocytes or 1 of 3 secretory cell lineages (Paneth, goblet, or enteroendocrine cells). In this study, we investigated the role of the Writ-responsive, Ets-domain transcription factor Spdef in the differentiation of goblet and Paneth cells. METHODS: The in vivo function of Spdef was examined by disrupting the Spdef gene in mice (Spdef(-/-) mice) and analyzing the intestinal phenotype using a range of histologic techniques and DNA microarray profiling. RESULTS: In accordance with expression data, we found that loss of Spdef severely impaired the maturation of goblet and Paneth cells and, conversely, led to an accumulation of immature secretory progenitors. Spdef appears to positively and negatively regulate a specific subset of goblet and Paneth cell genes, including Cryptdins, Mmp7, Ang4, Kallikreins, and MucZ CONCLUSIONS: Spdef acts downstream of Math1 to promote terminal differentiation of a secretory progenitor pool into Paneth and goblet cells.