Elongation factor 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. Elongation factor component of the little elongation complex (LEC), a complex required to regulate small nuclear RNA (snRNA) gene transcription by RNA polymerase II and III (PubMed:22195968, PubMed:23932780). Specifically required for stimulating the elongation step of RNA polymerase II- and III-dependent snRNA gene transcription (PubMed:23932780). ELL also plays an early role before its assembly into in the SEC complex by stabilizing RNA polymerase II recruitment/initiation and entry into the pause site. Required to stabilize the pre-initiation complex and early elongation. 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). Component of the little elongation complex (LEC), at least composed of ELL (ELL, ELL2 or ELL3), ZC3H8, ICE1 and ICE2. Interacts with AFF4; the interaction is direct. Interacts with EAF1 and EAF2. Interacts with ICE1 (via N-terminus domain). Interacts with ICE2. Interacts with USPL1 (PubMed:24413172). Colocalizes with EAF2 to nuclear speckles (PubMed:12446457). Colocalizes with coilin in subnuclear cajal and histone locus bodies (PubMed:12686606). Translocates in the LEC complex to cajal and histone locus bodies at snRNA genes in a ICE1-dependent manner. Associates to transcriptionally active chromatin at snRNA genes(PubMed:23932780). Expressed in all tissues tested. Highest levels found in placenta, skeletal muscle, testis and peripheral blood leukocytes. A chromosomal aberration involving ELL is found in acute leukemias. Translocation t(11;19)(q23;p13.1) with KMT2A/MLL1. The result is a rogue activator protein. Belongs to the ELL/occludin family.