Key component of the super elongation complex (SEC), a complex required to increase the catalytic rate of RNA polymerase II transcription by suppressing transient pausing by the polymerase at multiple sites along the DNA. In the SEC complex, AFF4 acts as a central scaffold that recruits other factors through direct interactions with ELL proteins (ELL, ELL2 or ELL3) and the P-TEFb complex. In case of infection by HIV-1 virus, the SEC complex is recruited by the viral Tat protein to stimulate viral gene expression. Interacts with ELL3; the interaction is direct (By similarity). Interacts with ELL2; the interaction is direct and leads to stabilize ELL2 and prevent ELL2 ubiquitination and degradation. Component of the super elongation complex (SEC), at least composed of EAF1, EAF2, CDK9, MLLT3/AF9, AFF (AFF1 or AFF4), the P-TEFb complex and ELL (ELL, ELL2 or ELL3). Interacts with ELL. Associates to transcriptionally active chromatin but not at snRNA genes. Ubiquitously expressed. Strongly expressed in heart, placenta, skeletal muscle, pancreas and to a lower extent in brain. Expressed in fetal heart, lung, brain and to a lower extent liver. A chromosomal aberration involving AFF4 is found in acute lymphoblastic leukemia (ALL). Insertion ins(5;11)(q31;q13q23) that forms a KMT2A/MLL1-AFF4 fusion protein. The disease is caused by variants affecting the gene represented in this entry. Belongs to the AF4 family. Contaminating sequence. Potential poly-A sequence.