CD55 expression distinguishes normal HSC from HSC with theAML1-ETOtranslocation. Bone marrow cells were isolated from an animal that was transplanted with cells expressing AML1-ETO and the green fluorescent protein (GFP) from a retroviral vector. Cells were stained for the HSC/progenitor cell phenotype (KLS) as well as for CD55. Cells that expressed AML1-ETO are shown below as GFP⁺ gated cells. Note that cells that express AML1-ETO express CD55 at approximately 100-fold greater levels on the cell-surface than GFP-negative (AML1-ETO-negative) bone marrow cells. MPC=myeloid progenitor cells of the phenotype c-Kit⁺ Lin⁻ Sea-1⁻ cells. HSC are defined c-Kit⁺ Lin⁻ Sca-1⁺ cells.