The super elongation complex (SEC) consists of the RNA polymerase II (Pol II) elongation factors eleven-nineteen Lys-rich leukaemia (ELL) proteins, positive transcription elongation factor b (P-TEFb) and several frequent mixed lineage leukaemia (MLL) translocation partners. It is one of the most active P-TEFb-containing complexes required for rapid transcriptional induction in the presence or absence of paused Pol II. The SEC was found to regulate the transcriptional elongation checkpoint control (TECC) stage of transcription, and misregulation of this stage is associated with cancer pathogenesis. Recent studies have shown that the SEC belongs to a larger family of SEC-like complexes, which includes SEC-L2 and SEC-L3, each with distinct gene target specificities.