miRNAs influence the expression of numerous proteins, including the expression of tumor suppressors and protooncogenes, thus becoming oncogenes and tumor suppressors themselves. As the same miRNA can have different targets in different tissues, its function will also be different in different types of tumors, depending on the intracellular milieu and the set of proteins for which its translation is modulated. Therefore, the same miRNA can act as a tumor suppressor and an oncogene in different tumors. Even in the same tumor, the same miRNA can be involved in regulation circles with feedback loops and potentially affect both tumor suppressors and oncogenes. As 50% of miRNA genes are located in regions associated with cancers, their expression is found to be deregulated in tumors. miRNAs were found to be members of signaling circuits, often involving also long non-coding RNAs (lnc RNAs) and circular RNAs (cRNAs).