Promoter activities were examined by comparing combinations of promoters and enhancers derived from various genes. A hybrid promoter comprising a combination of a CMV enhancer and a mammalian .beta.-actin promoter, or the post-transcriptional regulatory region of the genomic sequence Woodchuck Hepatitis Virus (WPRE) and a mammalian .beta.-actin promoter was found to be stronger than existing promoters. Furthermore, the activities of the .beta.-actin promoters could be enhanced by coexpressing the oncogene product Ras, which is a transactivator.