Intestinal stem cell (ISC) signaling maintains the balance of self-renewal and differentiation. Herein, the role of phosphatidylinositol 3-kinase (PI3K) signaling in ISC responses to radiation was interrogated using Villin-Cre pik3r1lox/lox (p85ΔIEC) mice and p85α-deficient human enteroids (shp85α). Lethal whole-body irradiation in mice was performed to monitor PI3K-mediated survival responses. Rectal biopsies from patients with radiation proctitis were examined by immunohistochemistry for th...