Leukemic cells differentiate in vivo in response to doxycycline. (A) Response of leukemic ME4b cells to doxycycline in vivo. Dot plots show Gr-1 and c-Kit expression (top panel), and staining with 7-amino-actinomycin D (7-AAD) and annexin V (bottom panel), on gated EGFP⁺ ME4b cells in the bone marrow of mice 18 days after transplantation with ME4b cells and 3 days after their treatment (right plot), or not (left plot), with doxycycline. Numbers in the plots represent the percentage of cells in each quadrant (top panel) and of apoptotic (7-amino-actinomycin D⁻annexin-V⁺) EGFP⁺ cells (bottom panel). The mean fluorescence intensity of Gr-1 expression by ME4b cells (top panel) increased from 684 to 1259 after doxycycline treatment. (B) Percentage of EGFP⁺ ME4b cells expressing c-Kit in the bone marrow and spleen of the recipient mice from panel A, treated (□) or not (■) with doxycycline. Bars represent mean values (n = 3); error bars represent SD.