Using an undifferentiated mouse CL6 cell line, DMSO was added to induce its differentiation into cardiac muscular cells in order to obtain gene fragments whose expression elevated upon the induction. The isolated gene had zinc finger domains and showed a significant homology to the Sp1 family genes. Furthermore, a human gene corresponding to this mouse gene was isolated. The protein encoded by this gene existed in the nucleus and bonded to a GC-box. The protein was revealed to repress the transcription regulatory activity of the CMV promoter and thus serves as a transcription factor.