Classifying microRNAs in cancer: the good, the bad and the ugly

MicroRNAs (miRNAs) have quite recently emerged as a novel class of gene regulators. Many miRNAs exhibit altered expression levels in cancer, and we are only starting to understand the functional consequences of the loss or gain of particular miRNAs to the cancerous phenotype. miRNAs can be classified with regard to their role in cancer as the Good, the Bad and the Ugly. The "Good", those miRNAs that are innocent bystanders in the oncogenic transformation process, whose expression profile might even be used for cancer diagnosis or prognosis. The "Bad", those miRNAs that are causally linked to tumorigenesis and directly modify tumor suppressor- or oncogenic- pathways. And the "Ugly", those miRNAs whose inappropriate loss or gain destabilizes the cellular identity of a tumor, which indirectly results in enhanced phenotypic variability and progression of the tumor. Hereunder we will discuss the possible ways in which miRNAs can be relevant to cancer biology, and possible experimental strategies for elucidating the mechanisms involved.