Objective To explore the mechanism of microRNA-378a-3p(miR-378a-3p)affecting the deve-lopment of breast cancer(BC)cells.Methods The expression of miR-378a-3p in BC cells based on the Cancer Genome Atlas Program database was analyzed.The starBase,miRDB,and miRWalk databases were used to predict the target genes of miR-378a-3p.Double-luciferase reporter experiments were performed to verify the targeted regulation of miR-378a-3p on NUAK family kinase 2(NUAK2).Real-time fluorescence quantitative PCR and Western blot analyses were used to detect the expression of miR-378a-3p and NUAK2 mRNA and protein in BC cells.Cell proliferation ability was detected through cell-proliferation experiments.Cell scratch assay and invasion assay were used to detect the migration and invasion abilities of cells,respectively.Apoptosis and cell-cycle experiments were conducted to detect cell apoptosis rate and cell cycle distribution.Results The expression of miR-378a-3p was significantly downregulated in BC cells,and miR-378a-3p overexpression inhibited the proliferation,migration,and invasion of BC cells.miR-378a-3p directly targeted NUAK2 and inhibited the mRNA and protein expression of NUAK2.Conclusion miR-378a-3p inhibits the proliferation,migration,and invasion of BC cells by targeting NUAK2.