Chemosensitizing effect of miR-21−/− in breast cancer cells. Knockdown of miR-21 by transfecting MDA-MB-468, MCF-7 and SKBR-3 cells with miR-21 inhibitor sensitizes them to chemotherapy drugs; (A) 90 nM miR-21 inhibitor causes a ~70% decrease in miR-21 expression in MDA-MB-468 cells compared to scramble, while 90 nM in MCF-7, and 30 nM in SKBR-3 cells, caused almost complete miR-21 inhibition compared to scramble. (B) At 48 h post-treatment, miR-21 inhibition combined with doxorubicin caused a ~40%, ~50%, and ~50% decrease in cell survival of MDA-MB-468, MCF-7, and SKBR-3, respectively, compared to scramble control. (C) At 48 h post-treatment, miR-21 inhibition combined with taxol caused a ~70%, ~53% and ~28% decrease in the cell survival of MDA-MB-468, MCF-7, and SKBR-3, respectively compared to scramble control. All the in vitro experiments were done in triplicates or quadruplets (n = 3/group), *** p < 0.001 or ** p < 0.01 or * p < 0.05, determined by two-way ANOVA with Bonferroni post-test or student’s t-test.