A method of detecting a polypeptide capable of regulating a transduction pathway and an expression library for use with such method are provided. The method is effected by introducing into cells endogenously expressing a trans acting regulator of the transduction pathway the expression library which includes a plurality of expression constructs each encoding a polypeptide and a reporter molecule expressible in the presence of a trans acting regulator of the transduction pathway. A level of expression within a predetermined range of the reporter is indicative of regulation of the transduction pathway by the polypeptide.