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Abstract: Introduction: MiRNAs regulate gene expression by binding to target sites and initiating translational repression and/or mRNA degradation. Studies have shown that miR-21 exerts its oncogenic activity by targeting the PDCD4 tumour suppressor 3′-UTR. However, the mechanism of this regulation is poorly understood. In colorectal cancer, loss of PDCD4 has been reported in association with increased tumour aggressiveness and poor prognosis. The purpose of this study was to delineate the interaction between PDCD4 and its oncogenic modulator miR-21 in colorectal cancer. Methods: A cohort of 48 colorectal tumours, 61 normal tissues and 7 polyps were profiled for miR-21 and PDCD4 gene expression. A subset of 48 specimens (31 tumours and 17 normal tissues) were analysed for PDCD4 protein expression by immunohistochemistry. Results: A significant inverse relationship between miR-21 and PDCD4 gene expression (p < 0.001) was identified by RT-qPCR. In addition, significant reduction of PDCD4 (p < 0.001) expression and reciprocal upregulation of miR-21 (p = 0.005) in a progressive manner from tumour-polyp-normal mucosae was identified. Analysis of protein expression by IHC revealed loss of PDCD4 staining in tumour tissue. Patients with disease recurrence had higher levels of miR-21. Conclusion: This study demonstrates the inverse relationship between miR-21 and PDCD4, thus suggesting that miR-21 post-transcriptionally modulates PDCD4 via mRNA degradation. Pharmacological manipulation of the miR-21/PDCD4 axis could represent a novel therapeutic strategy in the treatment of colorectal cancer. [Copyright &y& Elsevier]