Although in p210 t(9;22) ALL, most CD34+ CD38- CD19- as well as CD34+ CD38- CD19+ cells contained t(9;22) suggesting that the translocation is already present in the most primitive compartment, only the CD19+ subpopulation can give rise to leukemic engraftment whereas CD19- cells were capable of multilineage reconstitution. This evidence concerns the gene CD19 and acute lymphoblastic leukemia.