Cancer often spreads throughout the body via the bloodstream. However, once cancer cells have been transported to new locations, they have to find a way through the wall around blood vessels to invade new tissues. Researchers have now discovered that when blood platelets, the cells responsible for blood clotting, are activated, they release a molecule called ATP. ATP then binds to a protein called P2Y2 on the blood vessel wall, causing small openings to appear, which cancer cells can use to escape the blood vessel. Blocking ATP release from blood platelets inhibited cancer spread in mouse models of lung cancer and melanoma. Drugs that inhibit ATP release from platelets or block P2Y2 may therefore be able to curb cancer spread.