Potential mechanism of relapse after CD19-targeted immunotherapy in patients with Ph⁺ ALL and BCR-ABL fusion in the CD19⁺ leukemic compartment, as well as multipotent progenitor (MPP) cells and/or other hematopeietic stem cell (HSC) progenitor compartments. With anti-CD19 immune pressure and eradication of CD19⁺ leukemia, CD19^– myeloid phenotype leukemia can emerge, likely redifferentiated from the progenitor populations. Whether persistence of anti-CD19 pressure will increase the likelihood of this mechanism of resistance remains a question.