A modification of the two-hybrid system, called the one-hybrid system, is used for identifying proteins that can bind specific DNA sequences. In this method, the DNA sequence to be tested, the bait, is fused to a TATA box to drive expression of a reporter gene. The reporter gene is often chosen to complement a mutant phenotype; for example, a HIS gene may be used in a his⁻ mutant yeast strain. A cDNA library is constructed with the cDNA sequences translationally fused to the GAL4 activation domain and transformed into this yeast strain. Diagram how trans-acting proteins that bind to cis-acting regulatory sequences can be identified using a one-hybrid screen.