Clinical trials of an anti-CD19 CAR T-cell therapy for the treatment of acute lymphoblastic leukaemia has suggested that this might occur (Grupp et al., 2013) and, recently, data has been reported on a group of patients that initially showed a complete response but who subsequently relapsed owing to the presence of blast cells (Box 1) that no longer expressed CD19 (Maude et al., 2014). This evidence concerns the gene CD19 and acute lymphoblastic leukemia.