Likewise, 10,000 CAR T cells eradicated most CD19+ leukaemia blasts (initial response) but spared CD19− ones, which thus expanded unrestrainedly, reminiscent of clinical relapse cases driven by pre-existing CD19− populations (surface antigen loss) and selective pressure from CAR T cell (Supplementary Fig. 7a,b). Here, CD19 is linked to leukemia.