SEC inhibitor KL-1 (KL-1) is a potent and selective inhibitor of super elongation complex (SEC) that disrupts the interaction between the SEC scaffolding protein AFF4 and P-TEFb, resulting in impaired release of Pol II from promoter-proximal pause sites and a reduced average rate of processive transcription elongation. KL-1 exhibits an dose-dependent inhibitory effect on AFF4-CCNT1 interaction with Ki of 3.48 μM.