Immunohistochemistry showed low levels of miR-20a targets (BECN1, ATG16L1 and SQSTM1) in tumor tissues. However, γH2AX foci formation was dramatically increased in tumor tissues, whereas the staining of γH2AX in normal tissues was nearly undetectable (Figure 6c). In TCGA data set, breast cancer patients with higher levels of miR-20a show higher frequency of copy-number alterations and DNA mutations than cancer patients with lower miR-20a expression (Figure 8a), indicating that miR-20a promotes DNA damage response and increases genomic instability.