Proteins coded by homeobox-containing genes are sequence-specific DNA- binding proteins that have been implicated in the control of gene expression both in developing as well as in adult tissues. Two recently characterized human homeobox genes, HB24 and HB9, were found to be highly expressed in bone marrow cells enriched for CD34-positive cells, present at low levels in unfractionated bone marrow cells, and essentially undetectable in bone marrow cells depleted of CD34 cells. Treatment of CD34-enriched cells with recombinant interleukin-3 (IL-3) and granulocyte macrophage-colony-stimulating factor for 24 hours increased expression of HB24 threefold and HB9 fourfold. Based on studies with actinomycin D, the HB24 and HB9 transcripts in human CD34- positive cells have short half-lives, estimated to be 30 to 45 minutes. Downregulation of HB24 and HB9 expression was found following the treatment of in vitro cultures of CD34-positive cells with IL-3. Thus, the differentiation of CD34-positive cells along a specific cell lineage likely requires downregulation of both HB24 and HB9.

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