The human CD34 gene is expressed on early progenitor and stem cells in the bone marrow. Here we report the isolation of the human CD34 locus from a human P1 artificial chromosome (PAC) library and the characterization and evaluation of this genomic fragment for expression of reporter genes in stable cell lines and transgenic mice. We show that a 160-kb fragment spanning 110 kb of the 5′ flanking region and 26 kb of the 3′ flanking region of the CD34 gene directs expression of the human CD34 gene in the bone marrow of transgenic mice. The expression of human CD34 transgenic RNA in tissues was found to be similar to that of the endogenous murine CD34 gene. Colony-forming cell assays showed that bone marrow cells staining positive for human CD34 consist of early progenitor cells in which expression of CD34 decreased with cell maturation. In order to test the construct for its ability to express heterologous genes in vivo, we used homologous recombination in bacteria to insert the tetracycline-responsive transactivator protein tTA. Analysis of transgenic human CD34-tTA mice by cross breeding with a strain carrying Cre recombinase under control of a tetracycline-responsive element demonstrated induction of Cre expression in mice in a pattern consistent with the expression of the human CD34 transgene.