Inhibition of CDK9 signaling with the novel agent BAY 1143572 abrogates proliferation and induces apoptosis of ATL cells. (A) CDK9 phosphorylates RNA polymerase II (RNAPII) at serine 2 resulting in high levels of c-Myc and Mcl-1 and leading to proliferation and survival of ATL cells. (B) BAY 1143572 specifically inhibited CDK9-mediated phosphorylation at serine 2 of RNAPII resulting in decreased levels of c-Myc and Mcl-1, thus leading to growth arrest and apoptosis of ATL cells.