CD19 and acute lymphoblastic leukemia: Moreover, it is arguable that the epitope of the CD19 protein sequence recognized by the chimeric antigen receptor may be selectively deleted at progression in some patients with acute lymphoblastic leukemia who demonstrated an initial response to CD19 adoptive T-cell therapy (ACT), and the pre-existing CD19 isoforms with alternative splicing may be prone to acquired resistance [33, 34].