Origin of the brush cell lineage in the mouse intestinal epithelium

► The transcription factor Gfi1b defines the brush cell lineage and its progenitors. ► Enforced Notch signaling reduces but does not eliminate brush cell formation. ► Brush cell numbers increase dramatically in Atoh1-deficient epithelium. ► Brush cells are not a granulocytic (secretory) or columnar (absorptive) lineage. ► Hes1, Atoh1, and Gfi1b form a fate-determining Notch signaling driven ternary switch.