Mir-21 is upregulated in many tumors [47], including breast cancer [48], [49], and its role as a genuine oncogene has been recently shown in mice when inducible overexpression of miR-21 led to dramatic increase in hematological malignancies followed by their complete reversal after returning animals to a normal miR-21 expression mode [50]. The gene discussed is MIR21; the disease is breast carcinoma.