CD19 and acute lymphoblastic leukemia: CD19-negative relapses of pediatric BCP-ALL following blinatumomab treatment were first described in 2017 from a phase I/II study: four patients experienced CD19-negative relapse after prior blinatumomab-induced hematologic remission, and one patient showed CD19-negative progression during treatment after 10 days in cycle 1 with blasts showing a monocytic phenotype [40].