On the other hand, let-7a inhibits the proliferation of human glioblastoma (Lee et al., 2011), human nonsmall cell lung tumor (Johnson et al., 2007; Kumar et al., 2008; He et al., 2009), Burkitt lymphoma (Sampson et al., 2007), breast cancer cells (Yu et al., 2007), and primary fibroblasts (Legesse-Miller et al., 2009), through the repression of Ras, c-myc, E2F1, and CDC34 as well as elevation of p21. This evidence concerns the gene E2F1 and breast cancer.