Mediator is a protein complex which performs a very important role both for the transcription at the basal level which does not rely on transcription control factor and the transcription activating reaction that relies on the transcription control factor. It has the characteristic of binding to the 7 amino acid repeated structure (CTD) that exists in the C terminal of the biggest subunit of RNA polymerase II. 30 kinds of subunits have already being identified as its composing substance, but it is said that multiple kinds of complex with varied subunit compositions exist in the cells. Srb4p is one of the mediator subunit and is composed of 687 amino acid residues (aa). The product is prepared by immunizing rabbit with recombinant protein which was over-expressed in E. coli with a plasmid carrying the entire Srb4p protein (1-687aa) of budding yeast, and purified by chromatography. Using this antiserum in Western blotting, the band of 100 kD corresponding to Srb4p was obtained from the extract of yeast cells.