Human tumor cells in a pre-pre-B-ALL (first relapse) BM biopsy implanted subcutaneously for 19 weeks in the SCID mouse. BM cells from engrafted sample R19 were dispersed, stained, and analyzed by flow cytometry. The implant marrow cell population is homogeneous in terms of physical parameters (A, forward-scatter v side-scatter analysis) and virtually exclusively composed of viable (B, PE (fl2) v PI (fl3) signals) leukemic CD45⁺CD19⁺ cells (C, gates R1 + R2). (D), control isotype antibody staining.