**Abstract**

Various tools and techniques are being used for the diagnosis of cancer, but not a sole technique provides powerful result at the very early stages of cancer. This provides the need for type of tools which could detect cancer at early stages so that survival rate could be augmented. There are various diagnostic ways to identify cancer, but in each case, there are always circumstances to compromise on the sensitivity. In this framework, a new and more advanced approach of diagnosis for cancer is microRNA (miRNA). miRNAs are conserved regions among humans and animals, and their synthesis takes place in the nucleus and cytoplasm. There are several types of microRNAs that could be upregulated and downregulated in various cancers. A cancer cell could be identified by measurement of the expression pattern of miRNA. By examining the expression level for different types of cancers, miRNA can be used as biomarker for early detection of cancer in human beings.

**Keywords:** microRNA, cancer biomarker, diagnostic, colorectal, upregulation, downregulation, breast cancer, cervical, liver, prostate

#### **1. Introduction**

MicroRNAs (miRNAs) are a small, non-coding, single-stranded RNA consisting of around 22 nucleotides [1]. More than 3% of the human genome (gene portion) encodes for microRNA, and their number is around 1000 [2–4]. This small RNA can regulate gene expression posttranscriptionally [5–7]. This small RNA can regulate gene expression posttranscriptionally by binding to its cognate RNA target at the 3<sup>0</sup> untranslated region (UTR) [8–11]. A small microRNA was discovered for the first time in *C. elegans* and is encoded by the Lin-4 gene [12] providing evidence for its evolutionary conservation. This conserved microRNA was found to be involved in many important biological processes including cell proliferation, growth, apoptosis, etc. [13, 14], and many cell-based factors have been known to regulate its expression [15]. The genes transcribing the miRNA are considered to belong to the set of tumor suppressor genes, and the serum level of miRNA can be detected [16, 17]. There are certain miRNAs that can behave as either oncomiRs (whose expression can cause the cancer) or tumor suppressor depending on the context "Several miRNAs cannot be clearly and unequivocally categorized as tumor suppressors or oncomiRs because data in our hands are quite intricate and conflicting since they could act as tumor suppressors in one scenario or as oncomiRs in the other" [18].
