Overall, miR-21 functions as an oncomiR and modulates tumorigenesis through regulation of genes such as ANKRD46, BCL2, TIMP3, PDCD4, PTEN, TPM1, and MASPIN in breast cancer (Zhu et al., 2007; Frankel et al., 2008; Yang et al., 2009; Song et al., 2010; Wang et al., 2011; Yan et al., 2011). The gene discussed is PDCD4; the disease is breast cancer.