Abstract. The aim of this study was to determine the expression of miR-21, miR-31, miR-96 and miR-135b in 52 paired colorectal cancer (CRC) tissues and to analyze the correlation between microRNAs (miRNAs) and clinicopathological features. We developed a quantification method that relies on a standard plot, constructed from known concentrations of standards, in order to measure the number of miRNAs. In addition to this, we analyzed the expression levels of miR-21, miR-31, miR-96 and miR-135b in 52 cases of primary CRC and corresponding normal mucosal tissue using real-time PCR with SYBR-Green I. An independent sample t-test was used to compare the differential expression between tumor tissues and normal mucosal tissues. The Mann-Whitney U and Kruskall-Wallis tests were used to compare the correlation between miRNA expression levels and clinicopathological features. The expression of miR-21, miR-31, miR-96 and miR-135b was upregulated in the CRC tissues compared to normal mucosal tissues (P<0.05). Furthermore, miR-21 and miR-135b were positively correlated with the clinical stage (P=0.048 and P=0.029, respectively), while miR-96 and miR-135b were correlated with liver metastasis (P=0.006 and P=0.013, respectively). Our results suggest that miR-21, miR-31, miR-96 and miR-135b may function in the process of CRC development and progression. miR-135b levels in particular may correlate with the degree of malignancy. IntroductionColorectal cancer (CRC) is the third most common type of malignant neoplasm and the third leading cause of cancer-related mortalities worldwide (1). Conventional pathological diagnosis is traumatic, with the majority of cases being detected in the later stage. Early detection of CRC has puzzled clinicians and scientists for years. Even with annual fecal occult-blood testing, which has decreased the 13-year cumulative mortality rate from CRC by 33% (2), the outcome remains poor in patients with advanced disease. Only CRC diagnosed at an early stage is likely to be cured by surgical resection. Genetic alterations present in CRC, including those in APC, K-ras or p53, do not demonstrate a confirmed correlation between their mutation rate and clinical stage (3). Therefore, a reliable, sensitive and specific molecular diagnostic test for CRC is highly desirable from a clinical perspective.Accumulating evidence suggests that microRNAs (miRNAs) play a crucial role in the tumorigenesis and prognosis of cancer (4-7). miRNAs are non-coding, single-stranded RNAs of 18-25 nucleotides in length, which are able to regulate gene expression by inhibiting translation or decreasing stability of target mRNAs. Over the past few years, interest in the identification, detection and utilization of miRNA molecules has expanded rapidly. Bioinformatic analysis suggests that up to 30% of human genes may be regulated by miRNAs, despite the fact that they only constitute approximately 1% of the human genome (8,9).Changes in miRNA expression have been observed in a variety of human tumors, including breast (5), prostate (6),...