BRD4 and neoplasm: In this study, we provide inhibitory activity assays and structural and functional studies to demonstrate that 3478 is a potent selective inhibitor of BRD4–BD2 with a novel chemical scaffold and new binding mode to BRD4 bromodomains, and by suppressing c-Myc expression, 3478 reduced the human acute myeloid leukaemia (AML) MV4‐11 cell growth in vitro and xenografted tumour growth in mice without affecting body weight.