We propose a simple method for investigation of roles of individual transcription factors in complexes of multiple factors bound to the same cis element. By block-replacement mutagenesis, the whole cis element is replaced with a new one containing a binding site for a single factor. From the reporter activity of the mutant promoter construct, the importance of the individual factor in transcriptional activation is deduced. This approach allowed us to functionally identify SP1 as the most important PR1 binding transcription factor in the MN promoter.