Triple negative breast cancer (TNBC) is a major breast tumor subtype characterized by the lack of estrogen receptor (ER) and progesterone receptor (PR) expression, and the absence of human epidermal growth factor receptor 2 (HER2) amplification. TNBC patients have a poor clinical outcome with a high rate of recurrence at distant sites. Hence new therapeutic options are urgently needed. We have identified the prototypical BET bromodomain inhibitor JQ1 and BET bromodomain inhibitors (BBDis) in general as promising novel therapeutic agents in TNBC. Specifically, we have found that TNBC cell lines are significantly growth inhibited in cell culture by treatment with low nM range BBDis, compared with the luminal cell lines, which were relatively resistant. A potential BBDi drug target, BRD4, is highly expressed in TNBCs compared to luminal lines and its downregulation by Tet-inducible shRNA arrests TNBC cell growth. c-Myc, a transcription factor inhibited by JQ1 in some cell types, was also more abundant in TNBC lines compared to luminals, but its protein levels did not correlate with JQ1 response and did not change after JQ1 treatment. In line with the role of BRD4 in transcription restart after G2/M, JQ1 treatment prevented cell cycle re-entry, arrested TNBC cells in early G1, and induced luminal differentiation. The growth of established xenografts derived from TNBC cell lines and patient samples was efficiently inhibited by JQ1 treatment. Based on our preliminary data we hypothesize that (1) the downstream target(s) of BBDis critical for G1 re-entry in TNBC is not c-myc and remains to be identified, (2) targeting BRD4 or other bromodomain proteins using BBDis alone or in combination with other agents is an active novel therapeutic strategy in TNBC, and (3) BBDis will be an active therapy in metastatic TNBC. We propose three specific aims to test these hypotheses: Aim 1. To define and characterize the drug target and downstream targets of JQ1/BBDis in TNBCs. Aim 2. To conduct a clinical trial to test a BBDi in TNBC patients. Aim 3. To develop cell lines resistant to BBDis and characterize combination therapies to improve therapeutic responses and overcome acquired resistance.