CD19 and acute lymphoblastic leukemia: Alternatively spliced CD19 mRNA, along with a missense CD19 mutation or hemizygous deletions spanning the CD19 locus [84], were reported as the reason behind the absence of a full-length CD19 protein, as shown in Figure 5F. Downregulation of the CD19 antigen in B-ALL leukemic cells may be a signature of immunotherapy-resistant cells [85].