Migrating cancer cells seed in distant organs where they may remain in a dormant state for years before forming clinically detectable metastatic tumors. A new study from scientists at Mount Sinai shows dormant cancer cells assemble a type III collagen-enriched niche that is required to sustain tumor dormancy, as its disruption restores tumor cell division through the activation of STAT1 signaling. Using two-photon microscopy the authors show the dormancy-to-reactivation transition is accompanied by changes in the organization of the extracellular matrix and the abundance of type III collagen.