Real-time monitoring of changes in cells or cell products released from malignant lesions into the blood has opened new diagnostic avenues (“liquid biopsy”). In this issue of Cancer Cell, Best and colleagues describe that tumor-associated blood platelets provide specific information on the location and molecular composition of the primary tumor. Real-time monitoring of changes in cells or cell products released from malignant lesions into the blood has opened new diagnostic avenues (“liquid biopsy”). In this issue of Cancer Cell, Best and colleagues describe that tumor-associated blood platelets provide specific information on the location and molecular composition of the primary tumor.