(EN) The invention provides methods and compositions for identifying, i.e., selecting and/or screening for regulator molecules such as polypeptides and/or U1 SnRNAs which directly or indirectly influence, e.g., induce or suppress, the transcriptional activation of a target transcriptional regulatory region in a eukaryotic host cell. Also provided are regulator molecules identified by such methods, and methods of isolating polynucleotides encoding regulator molecules identified by these methods.