Neutrophils can capture and kill pathogens by releasing DNA and associated proteolytic enzymes into the extracellular space, forming structures known as neutrophil extracellular traps (NETs). Park et al. have shown that in the absence of infection, metastatic breast cancer cells can stimulate neutrophils to form metastasis-promoting NETs. Treatment with NET-digesting DNase I inhibited migration and invasion of breast cancer cells in vitro and reduced lung metastases in vivo when mice were treated with DNase I-coated nanoparticles. NETs were also observed in tumour samples from patients with triple-negative breast cancer, raising the possibility of targeting NETs to prevent metastasis.