Notch receptors transmit signals between adjacent cells. Signaling is initiated when ligand binding induces metalloprotease cleavage of Notch within an extracellular negative regulatory region (NRR). We present here the X-ray structure of the human NOTCH2 NRR, which adopts an autoinhibited conformation. Extensive interdomain interactions within the NRR bury the metalloprotease site, showing that a substantial conformational movement is necessary to expose this site during activation by ligand. Leukemia-associated mutations in NOTCH1 probably release autoinhibition by destabilizing the conserved hydrophobic core of the NRR.