A mutational analysis of Janus Kinase 2 (JAK2) revealed that the V617F mutation was present in granulocytes and in hematopoietic stem and progenitor cells (HSPCs), but not in the CD34+CD19+ population that mostly consists of Ph+ALL cells, indicating that this Ph+ALL clone did not originate from the ET clone carrying the JAK2-V617F mutation. Here, CD19 is linked to acute lymphoblastic leukemia.