To test whether BRCA1 expression is regulated by USP9X, endogenous USP9X was depleted using two independent siUSP9Xs in three breast cancer cell lines (MCF‐7, T47D, and MDA‐MB‐231) and HeLa cells, which express wild‐type BRCA1.44, 45 Then, mRNA and protein levels of USP9X and BRCA1 were examined using immunoblotting and qRT‐PCR analysis, respectively. Here, USP9X is linked to breast cancer.