
<ArticleTitle>Normal and leukemic <prot>CD34</prot>-negative human hematopoietic stem cells.</ArticleTitle>
<AbstractText>Considerable progress has been made in recent years in purifying human and murine hematopoietic stem cells. The essential marker identified is the sialomucin <prot>CD34</prot>, which is expressed on primitive cells and downregulated as they differentiate into more abundant mature cells. <prot>CD34</prot> is not unique to stem cells, however, as it is also expressed on clonogenic progenitors and some endothelial cells. Nevertheless, all clinical and experimental protocols are targeted to <prot>CD34+</prot> cells enriched by a variety of selection methods. Recent studies in both the murine and human systems have indicated that some stem cells capable of multilineage repopulation do not express detectable levels of cell surface <prot>CD34</prot>. These studies challenge the dogma that all human repopulating cells are found in the <prot>CD34+</prot> subset. However, the precise relationship between <prot>CD34-</prot> and <prot>CD34+</prot> stem cells is still not well understood. In this review, the results on the discovery of the <prot>CD34-</prot> repopulating cell are summarized and the impacts this discovery may have, both clinically and in our understanding of the organization of the human hematopoietic system, are examined.</AbstractText>
