Acute lymphoblastic leukaemia (ALL) is the most common kind of cancer in children. T-ALL, a subtype that resembles T-lymphocytes, can be treated successfully with the drug nelarabine. The drug has not been successful, however, with B-ALL, a subtype resembling B-lymphocytes. This has puzzled oncologists sinced the 1980’s. Now researchers have discovered the reason: B-ALL cells contain the enzyme SAMHD1, which deactivates the drug.