Coexpression of WT1 inhibits the AML1-ETO-expressing myeloid progenitors at more immature stages and lets them expand. One hundred thousand lineage-negative (Lin^-) cells of the AML1-ETO-transduced wild-type BM cell-derived colonies and AML1-ETO-transduced WT1-Tg BM cell-derived colonies were cultured in medium containing IL-3 and G-CSF for 5 days. (A) Proliferation of these cells in response to G-CSF + IL-3. Lin^- cells of the AML1-ETO-transduced wild-type BM cell-derived colonies (▪) and Lin^- cells of the AML1-ETO-transduced WT1-Tg BM cell-derived colonies (•). Numbers of viable cells were counted by the trypan blue exclusion method. The mean values ± SD of 5 independent assays for each group are shown. ^*P < .01; ^***P < .001. (B) May-Giemsa staining of these cells after culturing for 5 days (original magnification, × 1000). Stainings were analyzed using a Zeiss AxioSkop2 Plus microscope and a 100 ×/1.30 NA oil objective (both from Carl Zeiss, Oberkochen, Germany), and images were processed using AxioVision software version 3.0 (Carl Zeiss) and Adobe Photoshop CS. (Left) Lin^- cells of the AML1-ETO-transduced wild-type BM cell-derived colonies. Myelocytes and metamyelocytes can be seen. (Right) Lin^- cells of the AML1-ETO-transduced WT1-Tg BM cell-derived colonies. Myeloblasts with finely reticulated chromatin, 2 or more nucleoli, gray-blue cytoplasm, a few azurophilic cytoplasmic granules, and myeloblasts undergoing cell division can be seen.