Acts as a transcriptional transactivator of ELL and ELL2 elongation activities (By similarity). Acts as a transcriptional transactivator of TCEA1 elongation activity. 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 and ELL2 (By similarity). Isoform 1 and isoform 2 interact with TCEA1. Isoform 1 is expressed in ovary, uterus, mammary glands, brain, spleen, liver, lung, thymus, kidney, skeletal muscle, skin and testis. Isoform 2 is expressed in kidney. Expressed in brain and spinal cord at 10 dpc. Expressed in brain, spinal cord, cranial and spinal ganglia, lens, retina, cochlea, olfactory epithelium and pituitary at 12 dpc. Expressed in intestine, bladder endothelium, retinal ganglion cells, nephrons, bronchial epithelium, secretory epithelium of submandibular glands, tubular epithelium of the epididymis, ectodermal invaginations of mammary buds and vibrissae follicles, incisors and molars at 15 dpc. Belongs to the EAF family.