a Cytoplasmic HuR expression in breast cancer tissues with low or high grade. Patients with high-grade tumors (II-III or III, n = 42) have higher positively stained cytoplasmic HuR than those with low-grade tumors (II, n = 93) (*P = 0.0176, t-test). b Kaplan–Meier analysis of the distant disease-free survival of 134 patients comparing high and low cytoplasmic HuR. Patients with high cytoplasmic HuR have lower distant disease-free survival rate compared to those with low cytoplasmic HuR (**P = 0.0035, log-rank test). c, d Protein expression levels of HuR and downstream targets in MDA-MB-231 cells, cells with control sgRNA (sgControl) and two HuR KO clones. c Representative WB results from one experiment. d Quantified relative expression of HuR downstream target Bcl-2, Msi2, and XIAP. Values are mean ± SD from n = 3 independent experiments (***P < 0.001, two-way ANOVA). e Growth curves of MDA-MB-231 cells, sgControl and two HuR KO clones. Values are mean ± SD from n = 3 independent experiments (***P < 0.001, two-way ANOVA). f, g Colony formation of MDA-MB-231 cells, sgControl and two HuR KO clones. f colony numbers per well (***P < 0.001, one-way ANOVA, n = 3). g Representative images of colonies. h, i Invasion assay in parental MDA-MB-231 cells, sgControl and two HuR KO clones. h Representative images of stained invaded cells, scale bars: 200 μm. i The number of invaded cells per image (***P < 0.001, one-way ANOVA, n = 6). j, k Tumor initiation (j) and tumor growth (k) of MDA-MB-231 cells, sgControl and HuR KO1 clone in athymic nude mice. HuR KO1 is unable to engraft tumor in vivo (n = 10, ***P < 0.001, log-rank test for tumor initiation and two-way ANOVA for tumor growth).