miR-221 is an abnormally expressed in MML cells, and it induces the malignant phenotype through downmodulation of p27Kip1/CDKN1B and the c-KIT receptor; and the significantly increased level of miR-221 in patients’ circulating serum with MML is used as a new tumor marker, which is useful for the MML diagnosis, the in situ differentiating MML from stages I to IV MML, evaluating cancer progression, and monitoring patients during the follow-up period. The gene discussed is CDKN1B; the disease is neoplasm.