Normal bone marrow CD34⁺CD38⁻ cells can be divided into 2 distinct populations based on ALDH activity (high and low), with the ability to engraft immunodeficient mice restricted to the ALDH^high population. AML patient bone marrow contains an additional subpopulation with intermediate ALDH activity (ALDH^int); this population contains cells that bear the diagnostic leukemic cytogenetic marker and transplant leukemia into immunodeficient mice.