We have previously shown that the ability of ALL blasts to enter the CSF is a generic mechanism.12 Similar molecules such as PEG‐Asparaginase do not enter the CSF but do lead to a depletion of asparagine from the CSF in children with ALL.41 Pegylated gene vectors have been reported to enter the brain in CNS tumor models, although here the blood–brain barrier may be abnormal and the molecules are much smaller. Here, PAEP is linked to acute lymphoblastic leukemia.