We have placed a 225-bp fragment from the 5' end of the mouse rDNA transcription unit (from -169 to +56) in front of the SV40 tumor antigen coding sequence. After microinjection of this chimeric plasmid into nuclei of mouse L-cells expression of SV40 large T antigen has been observed. The expression of T antigen was dependent on the correct orientation of the rDNA fragment relative to the T antigen-coding region and was seen only in mouse cells. This indicates that the 225-bp rDNA fragment contains the sequence information required for pre-rRNA transcription and demonstrates for the first time that a protein-coding gene can be transcribed and expressed under the control of an RNA polymerase I promoter.