Metastasis occurs when cancer cells break away from the original tumor and travel through the bloodstream to form new tumors in other parts of the body. It is the leading cause of cancer-related death. Brain metastasis is particularly severe and affects 10–30% of patients with advanced lung, breast, and melanoma cancers.

While therapies exist for established brain tumors, there are limited strategies that directly target the very first cancer “seed cells” that enter and lodge in the brain.

Our brains, however, are equipped with immune cells called microglia that rapidly respond to pathogens and cancer cells by engulfing and digesting them. Yet scientists could not explain why microglia sometimes fail to destroy incoming seed cells because they could not watch this critical interaction in real-time in the living brain.