BRD4 and breast cancer: The role of BRD4 in cancer is largely dependent on the long isoform (BRD4-L), and we demonstrated by isoform-specific knockdown and endogenous protein detection as well as transgene expression that the less abundant short isoform of BRD4 (BRD4-L) S) is oncogenic and BRD4-L has a tumor suppressor role in breast cancer cell proliferation and migration as well as breast tumor formation and metastasis (128).