c-Myb is required for p210^BCR/ABL-dependent leukemogenesis. (A) Colony formation assays of p210^BCR/ABL p53^−/−c-Myb^ww or p53^−/−c-Myb^w/d marrow progenitors. Lin⁻Sca-1⁺Kit⁺ HSCs isolated from 5-FU–treated p53^−/−c-Myb^ww or p53^−/−c-Myb^w/d mice were retrovirally transduced with MigR1p210^BCR/ABL and, after GFP sorting, assessed for colony formation after plating in methylcellulose in the presence of a full dose or 1/10th of cytokines (IL-3, IL-6, and KL). Colonies were counted after 7 days. Representative of 3 different experiments performed in duplicate. P ≤ .001. (B) Survival of recipient mice injected with p210^BCR/ABL-transduced marrow cells Kaplan-Meier plot shows survival of C57BL/6J recipient mice after transplantation with p210^BCR/ABL p53^−/−c-Myb^w/w bone marrow cells (n = 14) or p210^BCR/ABL p53^−/−c-Myb^w/d bone marrow cells (n = 12). Bone marrow cells from 5-FU–treated (150 mg/kg, 4 days), p53^−/−c-Myb^w/w or p53^−/−c-Myb^f/d mice were retrovirally transduced with the MigR1p210^Bcr/Abl retrovirus and injected in lethally irradiated (1100 rad in 2 split doses of 550 each) syngenic C57BL/6J recipient mice. (C) Secondary leukemia in mice injected with GFP-positive cells isolated from mice with p210^BCR/ABL-induced primary leukemia. Kaplan-Meier plot shows survival of C57BL/6J recipient mice (n = 3) injected with GFP-positive cells from a mouse with leukemia induced by p210^BCR/ABL p53^−/−c-Myb^w/w cells or of recipient mice (n = 3) injected with GFP-positive cells from a mouse with leukemia induced by p210^BCR/ABL p53^−/−c-Myb^w/d cells. Sublethally irradiated (500 rad) recipient mice each received with 1.0 × 10⁵ GFP⁺ bone marrow cells.