• Akashi K, He X, Chen J, Iwasaki H, Niu C, Steenhard B, Zhang J, Haug J and Li L (2003). Transcriptional accessibility for genes of multiple tissues and hematopoietic lineages is hierarchically controlled during early hematopoiesis. Blood 101:383-9. Summary: Hematopoietic stem cells (HSCs) maintain hematopoiesis by giving rise to all types of blood cells. Recent reports suggest that HSCs also possess the potential to generate nonhematopoietic tissues. To evaluate the underlying mechanisms in the commitment of HSCs into multitissue and multihematopoietic lineages, we performed oligonucleotide array analyses targeting for prospectively purified HSCs, multipotent progenitors (MPPs), common lymphoid progenitors (CLPs), and common myeloid progenitors (CMPs). Here we show that HSCs coexpress multiple nonhematopoietic genes as well as hematopoietic genes; MPPs coexpress myeloid and lymphoid genes; CMPs coexpress myeloerythroid, but not lymphoid genes, whereas CLPs coexpress T-, B-, and natural killer-lymphoid, but not myeloid, genes. Thus, the stepwise decrease in transcriptional accessibility for multilineage-affiliated genes may represent progressive restriction of developmental potentials in early hematopoiesis. These data support the hypothesis that stem cells possess a wide-open chromatin structure to maintain their multipotentiality, which is progressively quenched as they go down a particular pathway of differentiation. Department of Cancer Immunology and AIDS, Dana-Farber Cancer Institute, Harvard Medical School, Boston, MA, USA.