miR-21 is overexpressed in a wide variety of cancers. It is believed to be an oncogene by downregulating the expression of many tumor suppressors, including programmed cell death 4 (PDCD4), tumor suppressor Pdcd4, the PTEN tumor suppressor gene, and the tumor suppressor gene tropomyosin 1 (TPM1). miR-21 is also found to be an important regulator of cancer-cell survival. Different from the survival pathway mediated by IL-6/Stat3 activation through the expression of antiapoptotic proteins, miR-21 is upregulated by Stat3, which mediates the suppression of apoptosis possibly through the inhibition of TPM1 and/or other proteins. Signosis� miR-21 plate assay kit provides a simple and quick method to analyze this miRNA molecule.