Transgenic constructs used to express heterologous genes in early hematopoietic progenitors. (A) The diagram at the top demonstrates the structure of the human CD34 PAC clone with 110 kb 5′ and 24 kb 3′ sequence, as well as the 8 exons encompassing 25 kb. (B) The structure of the tetracycline transactivator (tTA) cassette, which was inserted by homologous recombination in bacteria into human CD34 exon 1. Shown are the tTA cDNA followed by an internal ribosome entry site (IRES). The 2 different responder transgenic constructs, expressed from a tetracycline regulatory element (TRE) are shown as follows: (C) one that expresses Cre, and (D) a second expressing the human P210 BCR/ABL fusion protein. Details of these constructs and their function in mice have been published.^6-10