MiR-155 preleukemic transgenic mice are characterized exclusively by an expansion of the pre–B-cell population, whereas the MiR-155–induced leukemias display a mixed B220^low/IgM⁻ and B220^low/IgM⁺ immunophenotype. The myeloid lineage was equally expanded in all transgenics, preleukemic and leukemic alike. (A) Flow cytometry for B220 and IgM shows an increase of the B220⁺/IgM⁻ population in the preleukemic spleens compared with the wild-type ones; the leukemic spleens exhibit a mixed B220^low/IgM⁺ and B220^low/IgM⁻ immunophenotype. (B) Flow cytometry for CD11b shows an important increase of the myeloid lineage in the transgenic spleens (preleukemic and leukemic) compared with the wild-types.