Models of CD34 expression in bone marrow. (A) Diagram of conclusions from the current paper by Sato et al.13 The spheres represent arbitrary units of hematopoietic stem cell-repopulating activity, as measured by long-term bone marrow transplantation in mice. In normal murine bone marrow, the long-term repopulating activity is found entirely in the CD34^−/lowcompartment. During culture, the CD34⁻ stem cells expand and acquire CD34, and the CD34⁺ cells contain all detectable hematopoietic repopulating activity. After 5-FU treatment, this activity is distributed between CD34⁺ and CD34⁻ subsets of bone marrow. When bone marrow is taken from mice previously engrafted with CD34⁺ bone marrow derived from 5-FU–treated mice, the repopulating activity, measured in secondary recipients, is restricted to the CD34^−/low stem cell fraction. (B) Model of CD34 expression on murine hematopoietic stem cells. Resting stem cells from normal mice express little or no CD34, have a limited capacity for self-renewal, and can convert to a CD34⁺ phenotype upon activation. The CD34⁺ stem cells can self-renew as well as convert to a resting CD34⁻ phenotype. Alternatively, they may begin to differentiate, at which point they presumably lose their potential for self-renewal.