Intestinal stem cells (ISCs) and Paneth cells (PCs) reside at the bottom of the crypts of Lieberk & uuml;hn in the small intestine. Recent studies have shown that the transcription factor Mist1, also named BHLHA15, plays an important role in the maturation of PCs. Since there is an intimate interaction between PCs and ISCs, we speculated that the loss of Mist1 could impact these two neighboring cell types. Here, we report that mice lacking Mist1 had fewer but larger PCs with shrunken secretory granules, accompanied by an increase in goblet cells and tuft cells. Mist1 loss significantly decreased the number of proliferative crypt cells, especially columnar basal cells (CBCs). In addition, Mist1-deficient enteroids needed supplemental Wnt3a to support their growth. Results from RNA sequencing (RNA-seq) demonstrated an apparent deficiency of innate immunity in Mist1-knockout mice. Intriguingly, Mist1 loss increased the survival rate of mice subjected to whole abdominal irradiation (WAI). Moreover, radiation injury was ameliorated in Mist1-knockout mice compared with their wild-type littermates based on histological analysis and enteroid culture, which might be a consequence of increased contents of the endoplasmic reticulum (ER) and the increased activity of mTORC1 in Paneth cells. In summary, our data uncover that Mist1 plays an important functional role in PCs and regulates the maintenance of ISCs and their response to radiation injury. (c) 2024 The Pathological Society of Great Britain and Ireland.