PURPOSE: MicroRNA-21 (miR-21) is one of the miRNAs that are frequently and 
highly overexpressed in tumor tissue of colorectal cancer (CRC) patients; 
however, only a little is known about its functional role in CRC.
METHODS: We examined the expression level of miR-21 in 44 paired samples of 
tumoral and non-tumoral colon tissues diagnosed for CRC using TaqMan real-time 
PCR method. Furthermore, we used miR-21 inhibitor (anti-miR-21) to transient 
knockdown of miR-21 in DLD-1 colon cancer cells and examined the effects of 
miR-21 silencing on viability, apoptosis, chemosensitivity, cell cycle, and 
migration of DLD1 cells.
RESULTS: The expression levels of miR-21 were significantly increased in CRC 
tumor tissue (P < 0.0001). Significant differences in miR-21 levels were 
observed also between CRC tissues of patients with CRC in different clinical 
stages: I vs. II (P = 0.033) and I vs. IV (P = 0.021). Kaplan-Meier analysis 
proved that the miR-21 expression levels are correlated to shorter overall 
survival of CRC patients (P = 0.0341). MiR-21 silencing in DLD1 cell line had no 
effect on the cell viability; however, when combined with chemotherapeutics 
(5-FU, L-OHP, and SN38), it contributed to the decrease of cell viability. 
Suppression of miR-21 decreased cell migration ability of DLD-1 cells by nearly 
30 % (P = 0.016).
CONCLUSION: We have confirmed the overexpression of miR-21 in CRC samples and 
its correlation with advanced disease and shorter overall survival. These 
findings could be described in part by the fact that CRC cells with increased 
expression of miR-21 have higher migration ability.