A new diagnosis or treatment of cancer is provided. The present invention provides a protein Mimitin which is a target of a cancer gene myc protein, a variant thereof and a fragment thereof, as well as a polynucleotide molecule encoding them. An inhibitory substance of biological activity of the present invention, which is afforded by the binding of a polynucleotide molecule encoding a Mimitin protein and Myc provides a useful means for the diagnosis, prophylaxis or treatment of cancer.