The human serum response factor is a transcription factor belonging to the MADS domain protein family with members characterized from the plant and animal kingdoms. The X-ray crystal structure of the serum response factor core in a specific-recognition DNA complex shows that the functions of DNA binding, dimerization and accessory-factor Interaction are compactly Integrated into a novel protein unit. The intrinsic and induced conformation of the serum response element DNA is the principal DNA feature recognized in the specific complex.