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). A chromosomal aberration involving AFF1 is associated with acute leukemias. Translocation t(4;11)(q21;q23) with KMT2A/MLL1. The result is a rogue activator protein. Belongs to the AF4 family.