Phenotype of the myeloid leukemia cells. (A) Cytospins of cells in a representative tumor cell line (S1.1) with one PU.1 allele deleted and Arg235 mutated, showing the absence of differentiated cells but the presence in some cells of Sudan Black B–stained granules (arrows), a characteristic of the granulocyte lineage. (B) Flow cytometric plots showing expression on the leukemic cells of the granulocytic marker Gr-1 and the c-Kit cell surface receptor, a marker of early hematopoietic cells. Dotted lines indicate staining with isotype control antibodies. (C) Semiquantitative RT-PCR showing expression in the cells of the mRNA for myeloperoxidase and the receptors for G-CSF and GM-CSF.