Members of the miR-17-92 cluster promote BC cell growth, migration and invasion.37, 38 Recently, miR-17-92 members were also reported to downregulate MICA/B protein expression in ovarian tumors,23 glioma17 and other tumors,16 contributing to their immune escape. In our study, we provided new evidence that members of the miR-17-92 cluster and its paralogs miR-20a, miR-20b, miR-93 and miR-106b specifically downregulated not only MICA/B but also ULBP2 expression in BC and normal breast cell lines (Figure 2). Importantly, in BC cells, miR-20b is newly reported in NKG2DL regulation. Unlike other studies,17 we found that this miRNA-mediated downregulation of MICA/B and ULBP2 was partly due to the enhanced degradation of related mRNA transcripts. The 3'-UTRs of MICA and MICB contain binding sites for the tested miRNAs, which was verified in BC cells using a dual-luciferase reporter gene assay (Figures 3a and b). Interestingly, we first found that ULBP2 had no predicted binding sites for the tested miRNAs but could still be effectively regulated in BC cells. This miRNA-mediated ULBP2 inhibition may result in miRNA-induced MAPK/ERK signaling pathway downregulation (Figures 3c–g and Supplementary Figure 2d and e). Members of the miR-17-92 cluster were previously reported to downregulate the expression of the MAPK/ERK signaling pathway.27, 28 In this study, western blot and quantitative PCR showed that the protein and mRNA levels of ERK2 (MAPK1) were inversely regulated by miR-20a, miR-20b, miR-93 and miR-106b. The MAPK/ERK pathway was reported to influence NKG2DLs expression in some other tumors.39, 40 For instance, the overexpression of constitutively active ERK (especially ERK1) in ARK cells was found increased MICA/B and ULBP2 expression.25 We found that ERK2 inhibition effectively downregulated the expression of ULBP2 but not MICA/B, whereas ERK2 overexpression increased the ULBP2 expression (Figures 3f and g). Our study further demonstrated the functional importance of miRNA-dependent regulation of NKG2DLs in BC cells using NK cell cytotoxicity assays (Figure 4). A short-term lung clearance model confirmed that silencing NKG2DL-targeting miRNA contributed to immune recognition in vivo (Figure 5). These findings emphasize the functional significance of the tested miRNAs in MICA/B and ULBP2 regulation.