REXO1 or Elongin A-binding protein 1 (EloA-BP1) is an exonuclease domain-containing protein that can bind to Elongin. The Elongin complex stimulates the rate of transcription elongation by RNA polymerase II by suppressing the transient pausing of the polymerase at many sites along the DNA template. REXO1 is composed of 1221 amino acids and its mRNA is ubiquitously expressed. EloA-BP1 is capable of binding not only the NH(2)-terminal approximately 120 amino acid region of Elongin A, but also that of SII. Although REXO1 had no detectable effect on the rate of transcription elongation in vitro, it may play some role in the regulation of elongation in vivo.