POU4F1 is required for the malignant phenotypes of BLBC. A,B) The expression of POU4F1 in breast cancer cell lines and normal breast epithelial cell lines, measured by qRT‐PCR A) and western blotting B). C,D) Cell proliferation of 3 BLBC cell lines (BT549, HS578T, MDA‐MB‐231) transfected with POU4F1 siRNAs and control siRNA, detected by CCK8 cell proliferation assay C) and 5‐Ethynyl‐2'‐deoxyuridine (EdU) assay D). Scale bar, 50 µm. E) Representative images and quantitation of the colony formation of 3 BLBC cell lines transfected with POU4F1 siRNAs and control siRNA. F) Representative images and quantitation of the anchorage‐independent growth assays in soft agar of MDA‐MB‐231 cells after shRNA‐mediated knockdown of POU4F1. Scale bar, 50 µm. G) Representative images and quantitation of cell invasion of 3 BLBC cell lines transfected with POU4F1 siRNAs and control siRNA, assessed through Boyden chambers assay. Scale bar, 50 µm. H–J) Tumor growth curves H), tumor images I) and tumor weight on day28 J) of xenografts of MDA‐MB‐231 cells transduced with shRNA targeting POU4F1 or scramble. Data were presented as mean ± S.D. A,C–H,J), n = 3 A,C–G) and n = 6 H–J). ***P < 0.001 compared with siCtrl C–E,G) and scramble F,H,J) by two‐tailed one‐way ANOVA and Dunnett's multiple‐comparisons test.