While we can readily identify FGF targets genes, to address the deficiency in our knowledge of what why genes respond to FGF signaling, we have initiated a project to molecularly define the cis-acting elements that cause FGF targets genes to respond to FGF signals. One arm of this project involves characterizing the elements that regulate the expression of Sprouty2 and Sprouty4, two well-characterized FGF target genes that encode proteins that regulate the FGF signaling pathway. In our preliminary studies we have defined sequences derived from Sprouty2 that drive expression in key regions of embryonic FGF signaling. Using mutants described in project ZIA BC 010338 (The Role of Fgf Signaling in Vertebrate Development ) we have determined that reporter gene expression, driven by these sequences, is responsive to FGF signals, in the developing mouse embryo. Transfection experiments reveal that these sequences are also FGF responsive in culture cells. Now the goals are to a) determine the minimal FGF b) determine the trans-activing factors that bind to these sequences, using DNA-based techniques. We will also use these elements, as positive controls, define a cell culture system in which to assay FGF responsiveness. Such a system will be used, in our second arm of this project, to screen novel libraries to define regions of FGF responsiveness in the mouse genome. We will always validate any in vitro insights with in vivo experiments, thus providing a powerful approach that should guarantee the significance of our data.