Change in leukemic blast expression of CD19 after selective pressure of blinatumomab. (A) Multicolor flow cytometric analysis of leukemia blasts after conventional chemotherapy exposure demonstrates predominantly normal CD19 expression. (B) After exposure to the CD19-directed immunotherapeutic agent blinatumomab, there is a large CD19⁻ blast population. This correlates with the progression of disease seen during administration of blinatumomab, possibly as a result of hypermutation of the malignant clone.