TAT-PiET is a cell-penetrating peptide targeting the extra-terminal (ET) domain of BRD4 that disrupts the BRD4/JMJD6 interaction. BRD4/JMJD6 is a protein complex that is critical for oncogene expression and breast cancer , and TAT-PiET inhibits BRD4/JMJD6 target gene expression and breast cancer cell growth. TAT-PiET also resists endocrine resistance in ERα-positive breast cancer cells and is a potential inhibitor of breast cancer.