Background and Aims: The aberrant expressions of microRNAs (miRNAs) have been demonstrated to be closely associated with the occurrence and development of malignant tumors. The expressions of some miRNAs have clear correlations with the invasive clinicopathologic characteristics of papillary thyroid carcinoma (PTC). This study was conducted to investigate the expression of miR-221 in PTC and its influence on the biological behaviors of PTC cells.  
Methods: The expressions of miR-221 in 51 paired samples of PTC and adjacent tissue were detected by qPCR. In PTC K1 cells after transfection with miRNA random sequence (negative control group) or miR-221 inhibitor (miR-221 inhibitor group) with the untreated K1 cells as blank control, the proliferation was detected by MTT colorimetry, apoptosis and cell cycle were analyzed by flow cytometry, and invasion ability were determined by Transwell chamber.
Results: The relative expression level of miR-221 in PTC tissue was significantly higher than that in the adjacent tissue (P<0.05). In K1 cells of miR-221 inhibitor group compared with blank control group, the proliferative ability was significantly reduced; the apoptosis rate was significantly increased and the proportion of G0/G1 phase was increased while the proportion of G2/M phase decreased significantly; the invasion capability was significantly decreased, and all the differences had statistically significance (all P<0.05). there were no significant differences in above studied indexes between negative control group and blank control group (all P>0.05).
Conclusion: The expression of miR-221 is increased in PTC, which may probably affect the proliferation and invasion capability of PTC cells through regulating the cell cycle and apoptosis. So, miR-221 has a potential application value as a biomarker for early diagnosis and treatment of PTC.