Experimental approach to define the domain structure of regulatory factors. (a) Reporter gene construct containing a UAS responsive to the yeast transcription factor Gal4. (b) Wildtype and mutant forms of Gal4 tested for binding to the UAS and for stimulating expression of the reporter gene. The DNA-binding domain is at the amino terminus of Gal4, and the activation domain is at the carboxy terminus of Gal4.