We propose to examine the role of specific DNA sequences, and the effect of the position of these sequences within the genome on the developmental regulation of gene expression. Cloned fragments of mouse mammary tumor virus (MTV), whose transcription is induced by glucocorticoids, will be introduced into mouse zygotes by microinjection. The injected embryos will be implanted into foster mothers, and adult mice derived from these zygotes. These mice will then be bred to give offspring with exogenously introduced copies of MTV at different chromosomal loci. Southern blot analysis will be used to identify mice that contain newly acquired sequences, while Northern blot analysis of RNA isolated from tissues of these mice will be used to determine in which tissues the viral sequences are expressed. These exogenously introduced DNAs will contain: sequences from the whole provirus; proviruses containing deletions genetically engineered in vitro; proviral elements to ligated the HSV-1 thymidine kinase gene; or proviral elements ligated to murine and human c-myc genes. Our goal is to determine whether the viral sequences and/or the chromosomal position of MTV determines its steroid-regulated expression in mammary tissue and whether an exogenously introduced sequence will be expressed in non-mammary tissue. We will also determine whether the MTV sequences can exert an effect on nonviral DNA in a hormonal and developmentally regulated fashion. Because MTV gene expression is hormonally controlled and is restricted to lactating mammary glands and tumors in mice, the approach described will lead to an understanding of the mechanisms involved in the development of regulation of a steroid inducible gene.