An object of the present invention is to provide DNA which regulates the expression of miR-140, a reporter vector which contains the DNA, cells and animals into which the reporter vector is introduced and a screening system for drugs which control the expression of miR-140 and is also to contribute in the development of new therapeutic agents for cartilage diseases such as osteoarthritis and intervertebral disk degeneration using the screening system. The DNA sequence according to the present invention is able to express any gene in a site where miR-140 is expressed and, in addition, it is also able to be utilized for screening a drug which regulates the expression of miR-140. Moreover, the drug that is selected by the screening system of the present invention is expected as a therapeutic agent for cartilage diseases accompanied by abnormality of cartilage.