SEC inhibitor KL-1 (KL-1), a peptidomimetic lead compound, is a potent, selective super elongation complex (SEC) inhibitor and 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. SEC inhibitor KL-1 exhibits an dose-dependent inhibitory effect on AFF4-CCNT1 interaction with a Ki of 3.48 μM.IC50 & Target: SEC