MK is a new identified cytokine, which belongs to the heparin binding family. MK molecule is a small polypeptide whose gene expresses only in mid-gestation embryo and adult kidney.The expression of MK gene has also been shown in various carcinoma cells. MK can promote the growth and differentiation of normal cells,particularly the development of nerve cells.MK can also inhibit the growth of several kinds of carcinoma cells.The reason why MK gene is expressed in adult kidney has not been elucidated.