The super elongation complex (SEC) is extremely important in the transcriptional elongation checkpoint control stage of transcription and is composed mainly of P-TEFb (positive transcription elongation factor), ELL (11–19 lysine-rich leukemia gene) family proteins, AFF (AF4/FMR2) family proteins, ENL (11–19 leukemia), AF9 (ALL1-fused gene from chromosome 9), and many other transcription factors (Luo et al., 2012b). Here, AFF1 is linked to leukemia.