In a patient with chronic lymphocytic leukemia responding to CTL019 therapy, peripheral expansion of a single T-cell clone with disruption of the TET2-gene due to CD19-CAR transgene integration (and hypomorphic mutation in the second allele) was thought to be the predominant mediator of the observed antitumor response36. The gene discussed is TET2; the disease is B-cell chronic lymphocytic leukemia.