**4.1 miRNAs in colorectal cancer diagnosis**

Early diagnosis is essential for CRC patients since they have more favorable prognosis. Fecal blood test and colonoscopy techniques are being currently used for early screening. However, fecal blood tests are not very efficient for detecting early carcinoma formation. Colonoscopy is a gold standard technique, it reduces cancer risk about 30–75%, yet it is invasive and expensive technique and highly uncomfortable for a patient [56]. Therefore, noninvasive and inexpensive screening and diagnostic methods or biomarkers are needed. miRNAs are promising candidates for noninvasive biomarker diagnosis. Diagnostic miRNAs can be isolated from blood or stool samples as well as tumor tissues [57] (**Table 1**).

There are different miRNA profiling studies comparing CRC samples with normal healthy tissue samples; however, each study emphasized on different set of miRNAs in CRC diagnosis and progression. According to miRNA profile study, miR-18a, -20a, -21, -29a, -92a, -106b, -133a, -143, and -145 expression levels were found to be significantly changed in CRC patients when compared with normal patients, and these markers can be used for CRC diagnosis [59]. In a systematic review, miR-106a, -30a-3p, -139, -145, -125a, and -133a were proposed as diagnostic biomarkers [60]. In another study, miR-143, -145, -21, -320, -126, -484-5p, -143, -145, -16, -125b, -21, and -106 were found to be candidate for diagnostic biomarkers [57]. While studies share some common miRNAs (such as miR143, miR145, miR106, miR21), they are differing in their list of miRNAs. In fact, the type of miRNAs can be differed due to the type of sample (blood or stool), experimental procedures, and used microRNA platforms. Another handicap of these studies is that they have been conducted with a small number of samples. Larger sample studies and additional meta-analyses are need for better determination of CRC-related diagnostic markers. Still, it can be said that miRNAs are very promising noninvasive markers for tumor diagnosis.
