For example, in a Phase II clinical trial investigating pediatric and adolescent patients with relapsed or refractory B-cell acute lymphoblastic leukemia (B-ALL), only one patient bearing CD19-positive leukemia cells relapsed, while fifteen cases of CD19-negative recurrence were confirmed, which is an important issue in achieving complete remission of the disease [24]. Here, CD19 is linked to B-cell acute lymphoblastic leukemia.