CD19 and acute lymphoblastic leukemia: Purple, B-ALL; green, non-malignant pre-B cells (CD19+CD93+IgM-cKit-) in bone marrow (BM) of wild-type (WT) mice transplanted with B-ALL cells; blue, non-malignant pre-B cells in BM of WT mice (no B-ALL); filled gray, non-malignant Ltb-deficient pre-B cells in BM of Ltb-/- mice.