The present invention relates to an isolated Period 1 (Per1) promoter DNA inducing rhythmical expression of a gene operably linked thereto. This invention also provides a DNA comprising a Period 1 promoter DNA and a gene operably linked thereto, the gene being under the regulation of the promoter DNA. The present invention further provides transformants and transgenic mammals into which the DNA has been introduced. The transformants and transgenic mammals are useful in the screening of pharmaceutical drugs against diseases and disorders pertaining to the circadian rhythm.