Flow cytometry using mouse anti-human ROR1 mAb 2A2 (Fig. 2A and B) or goat anti-human ROR1 pAbs (data not shown) revealed cell surface ROR1 expression in 5 pre-B-ALL cell lines, including all 4 E2A-PBX1+ cell lines and 1 out of 3 MLL-rearranged cell lines. Here, PBX1 is linked to acute lymphoblastic leukemia.