A deadly feature of acute lymphoblastic leukemia (ALL) is its invasion of the central nervous system. ALL in the central nervous system is very difficult to treat, because most drugs are blocked from the organ system due to a "blood-brain barrier" designed to protect the brain. How cancer cells enter the central nervous system has been an unanswered question for researchers and clinicians for decades. A research team has now found that this blood cancer infiltrates the central nervous system not by breaching the blood-brain barrier, but by evading the barrier altogether.