Our results that knockout of BRD4 significantly blocked colony formation in the TNBC cells lines MDA-MB-231, MDA-MB-468, and HS578T carrying mutant p53, while it only slightly decreased the colony formation of MCF-7 cells that carry wild-type p53, suggesting that the sensitivity of the breast cancer cells’ response to the BET inhibitor might be dependent on the status of p53. This evidence concerns the gene BRD4 and breast cancer.