A research group from the Children's Hospital of Philadelphia (CHOP) investigated flow cytometric data from 628 cases of relapsed or refractory B-ALL and showed that before treatment, approximately 17% of cases had CD19-negative tumor cells (defined as the presence of more than 1% of negative cells), which may lead to relapse after anti-CD19 CAR T-cell therapy. The gene discussed is CD19; the disease is precursor B-cell acute lymphoblastic leukemia.